A 53-year-old man presented to the emergency department with severe lower back pain, saddle anesthesia, and urinary dysfunction. He had undergone endoscopic lumbar interbody fusion for highly migrated lumbar disk herniation and lumbar instability 10 days ago. Emergency computed tomography showed that the bone graft materials had migrated to the sacral canal, and a mass with low intensity was seen located at the end of the dural cavity on T2-weighted magnetic resonance imaging. It was suspected that the bone graft materials had migrated into the dural cavity at the operative level and fallen into the end of the dural cavity due to gravity, causing acute cauda equina syndrome (CES). After emergency durotomy, the bone graft materials were completely removed. At the 18-month follow-up, the patient recovered without further complications. This procedure resulted in a rare case of CES caused by intradural bone graft after endoscopic lumbar interbody fusion. Spine surgeons should be aware of this rare but potentially dangerous complication, especially in water-based endoscopic lumbar interbody fusion. This case report shows that early recognition and prompt treatment can significantly improve the symptoms of CES, including saddle numbness and bladder dysfunction. [Orthopedics. 20XX;XX(X):xx-xx.].