diff --git a/python/sglang/jit_kernel/diffusion/triton/causal_conv3d_pad.py b/python/sglang/jit_kernel/diffusion/triton/causal_conv3d_pad.py
new file mode 100644
index 000000000..5846a1084
--- /dev/null
+++ b/python/sglang/jit_kernel/diffusion/triton/causal_conv3d_pad.py
@@ -0,0 +1,122 @@
+from __future__ import annotations
+
+import torch
+import triton  # type: ignore
+import triton.language as tl  # type: ignore
+
+
+@triton.jit
+def _fused_cat_pad_5d_kernel(
+    x_ptr,
+    cache_ptr,
+    out_ptr,
+    total,
+    channels,
+    t_size,
+    h_size,
+    w_size,
+    cache_t,
+    out_t,
+    out_h,
+    out_w,
+    pad_d_left,
+    pad_h_top,
+    pad_w_left,
+    block_size: tl.constexpr,
+):
+    offsets = tl.program_id(0) * block_size + tl.arange(0, block_size)
+    mask = offsets < total
+
+    ow = offsets % out_w
+    tmp = offsets // out_w
+    oh = tmp % out_h
+    tmp = tmp // out_h
+    out = tmp % out_t
+    tmp = tmp // out_t
+    oc = tmp % channels
+    ob = tmp // channels
+
+    iw = ow - pad_w_left
+    ih = oh - pad_h_top
+    src_t = out - pad_d_left
+
+    valid = (
+        mask
+        & (iw >= 0)
+        & (iw < w_size)
+        & (ih >= 0)
+        & (ih < h_size)
+        & (src_t >= 0)
+        & (src_t < cache_t + t_size)
+    )
+    from_cache = src_t < cache_t
+
+    x_t = src_t - cache_t
+    clamped_iw = tl.minimum(tl.maximum(iw, 0), w_size - 1)
+    clamped_ih = tl.minimum(tl.maximum(ih, 0), h_size - 1)
+    clamped_x_t = tl.minimum(tl.maximum(x_t, 0), t_size - 1)
+    clamped_src_t = tl.minimum(tl.maximum(src_t, 0), cache_t - 1)
+
+    x_offsets = (
+        ((ob * channels + oc) * t_size + clamped_x_t) * h_size + clamped_ih
+    ) * w_size + clamped_iw
+    cache_offsets = (
+        ((ob * channels + oc) * cache_t + clamped_src_t) * h_size + clamped_ih
+    ) * w_size + clamped_iw
+
+    x_vals = tl.load(x_ptr + x_offsets, mask=valid & ~from_cache, other=0.0)
+    cache_vals = tl.load(cache_ptr + cache_offsets, mask=valid & from_cache, other=0.0)
+    vals = tl.where(from_cache, cache_vals, x_vals)
+    tl.store(out_ptr + offsets, vals, mask=mask)
+
+
+def fused_causal_conv3d_cat_pad(
+    x: torch.Tensor,
+    cache_x: torch.Tensor,
+    padding: list[int] | tuple[int, ...],
+) -> torch.Tensor:
+    width_left, width_right, height_top, height_bottom, depth_left, depth_right = (
+        padding
+    )
+    depth_left -= cache_x.shape[2]
+    assert depth_left >= 0
+    assert depth_right == 0
+    assert width_left == width_right
+    assert height_top == height_bottom
+
+    bsz, channels, t_size, h_size, w_size = x.shape
+    cache_t = cache_x.shape[2]
+    out = torch.empty(
+        (
+            bsz,
+            channels,
+            t_size + cache_t + depth_left + depth_right,
+            h_size + height_top + height_bottom,
+            w_size + width_left + width_right,
+        ),
+        device=x.device,
+        dtype=x.dtype,
+    )
+    block_size = 256
+    total = out.numel()
+    grid = (triton.cdiv(total, block_size),)
+    with torch.get_device_module().device(x.device):
+        _fused_cat_pad_5d_kernel[grid](
+            x,
+            cache_x,
+            out,
+            total,
+            channels,
+            t_size,
+            h_size,
+            w_size,
+            cache_t,
+            out.shape[2],
+            out.shape[3],
+            out.shape[4],
+            depth_left,
+            height_top,
+            width_left,
+            block_size,
+        )
+    return out
diff --git a/python/sglang/multimodal_gen/runtime/layers/parallel_conv.py b/python/sglang/multimodal_gen/runtime/layers/parallel_conv.py
index 068a6b925..780d354c9 100644
--- a/python/sglang/multimodal_gen/runtime/layers/parallel_conv.py
+++ b/python/sglang/multimodal_gen/runtime/layers/parallel_conv.py
@@ -14,6 +14,13 @@ from sglang.multimodal_gen.runtime.distributed.parallel_state import (
 )
 from sglang.multimodal_gen.runtime.platforms import current_platform
 
+if current_platform.is_cuda():
+    from sglang.jit_kernel.diffusion.triton.causal_conv3d_pad import (
+        fused_causal_conv3d_cat_pad,
+    )
+else:
+    fused_causal_conv3d_cat_pad = None
+
 _SPATIAL_PARALLEL_DECODE_DISABLED = contextvars.ContextVar(
     "spatial_parallel_decode_disabled", default=False
 )
@@ -58,6 +65,49 @@ def _tensor_chunk(x: torch.Tensor, dim: int = -2, world_size: int = 1, rank: int
     )
 
 
+def _can_fuse_causal_conv3d_cat_pad(
+    x: torch.Tensor,
+    cache_x: torch.Tensor | None,
+    padding: list[int],
+) -> bool:
+    if cache_x is None or fused_causal_conv3d_cat_pad is None:
+        return False
+    if not x.is_cuda or not x.is_contiguous() or not cache_x.is_contiguous():
+        return False
+    if x.dim() != 5 or cache_x.dim() != 5 or x.dtype != cache_x.dtype:
+        return False
+    if x.shape[0] != cache_x.shape[0] or x.shape[1] != cache_x.shape[1]:
+        return False
+    if x.shape[3:] != cache_x.shape[3:]:
+        return False
+
+    width_left, width_right, height_top, height_bottom, depth_left, depth_right = (
+        padding
+    )
+    if width_left != width_right or height_top != height_bottom or depth_right != 0:
+        return False
+    if depth_left < cache_x.shape[2]:
+        return False
+    return bool(width_left or height_top)
+
+
+def causal_conv3d_cat_pad(
+    x: torch.Tensor,
+    cache_x: torch.Tensor | None,
+    padding: list[int],
+) -> torch.Tensor:
+    if cache_x is not None and padding[4] > 0:
+        if cache_x.device != x.device:
+            cache_x = cache_x.to(x.device)
+        if _can_fuse_causal_conv3d_cat_pad(x, cache_x, padding):
+            return fused_causal_conv3d_cat_pad(x, cache_x, padding)
+        x = torch.cat([cache_x, x], dim=2)
+        padding[4] -= cache_x.shape[2]
+    if any(padding):
+        x = F.pad(x, padding)
+    return x
+
+
 def split_for_parallel_decode(
     x: torch.Tensor, upsample_count: int, world_size: int, rank: int
 ):
@@ -551,12 +601,7 @@ class SpatialParallelCausalConv3d(nn.Conv3d):
         if spatial_parallel_decode_disabled():
             padding[2] = self.height_pad_top
             padding[3] = self.height_pad_bottom
-        if cache_x is not None and self._padding[4] > 0:
-            cache_x = cache_x.to(x.device)
-            x = torch.cat([cache_x, x], dim=2)
-            padding[4] -= cache_x.shape[2]
-
-        x = F.pad(x, padding)
+        x = causal_conv3d_cat_pad(x, cache_x, padding)
         x = x if current_platform.is_amp_supported() else x.to(self.weight.dtype)
 
         if spatial_parallel_decode_disabled():
diff --git a/python/sglang/multimodal_gen/runtime/models/vaes/autoencoder_kl_qwenimage.py b/python/sglang/multimodal_gen/runtime/models/vaes/autoencoder_kl_qwenimage.py
index cad3530f1..ea7fff1fa 100644
--- a/python/sglang/multimodal_gen/runtime/models/vaes/autoencoder_kl_qwenimage.py
+++ b/python/sglang/multimodal_gen/runtime/models/vaes/autoencoder_kl_qwenimage.py
@@ -24,6 +24,7 @@ from sglang.multimodal_gen.runtime.layers.parallel_conv import (
     SpatialParallelCausalConv3d,
     SpatialParallelConv2d,
     SpatialParallelZeroPad2d,
+    causal_conv3d_cat_pad,
     chunk_height_for_parallel_decode,
     disable_spatial_parallel_decode,
     gather_and_trim_height,
@@ -87,11 +88,7 @@ class QwenImageCausalConv3d(nn.Conv3d):
 
     def forward(self, x, cache_x=None):
         padding = list(self._padding)
-        if cache_x is not None and self._padding[4] > 0:
-            cache_x = cache_x.to(x.device)
-            x = torch.cat([cache_x, x], dim=2)
-            padding[4] -= cache_x.shape[2]
-        x = F.pad(x, padding)
+        x = causal_conv3d_cat_pad(x, cache_x, padding)
         return super().forward(x)
 
 
diff --git a/python/sglang/multimodal_gen/runtime/models/vaes/wanvae.py b/python/sglang/multimodal_gen/runtime/models/vaes/wanvae.py
index 0f3e2f424..1bbad1241 100644
--- a/python/sglang/multimodal_gen/runtime/models/vaes/wanvae.py
+++ b/python/sglang/multimodal_gen/runtime/models/vaes/wanvae.py
@@ -40,6 +40,7 @@ from sglang.multimodal_gen.runtime.layers.parallel_conv import (
     SpatialParallelCausalConv3d,
     SpatialParallelConv2d,
     SpatialParallelZeroPad2d,
+    causal_conv3d_cat_pad,
     chunk_height_for_parallel_decode,
     disable_spatial_parallel_decode,
     gather_and_trim_height,
@@ -217,11 +218,7 @@ class WanCausalConv3d(nn.Conv3d):
 
     def forward(self, x, cache_x=None):
         padding = list(self._padding)
-        if cache_x is not None and self._padding[4] > 0:
-            cache_x = cache_x.to(x.device)
-            x = torch.cat([cache_x, x], dim=2)
-            padding[4] -= cache_x.shape[2]
-        x = F.pad(x, padding)
+        x = causal_conv3d_cat_pad(x, cache_x, padding)
         x = (
             x if current_platform.is_amp_supported() else x.to(self.weight.dtype)
         )  # casting needed if amp isn't supported