With the advancement of cancer treatment technologies, immunotherapy has begun to be widely utilized. Colon cancer is one of the most common types of cancer, with metastasis being a frequent occurrence in late-stage patients. Hence, immunotherapy, as an emerging and potentially effective treatment modality, merits exploration to enhance patient survival rates and clinical benefits. However, various immune-related adverse events cannot be entirely avoided. Myasthenia gravis induced by immunotherapy serves as a rare but potentially lethal adverse event, and it has been increasingly reported. Understanding the mechanisms of irAEs can aid in controlling the side effects induced by treatment. Here, we reported a case of myasthenia gravis occurring after anti-PD-1 therapy for late-stage colon cancer.