Small cell carcinoma of the prostate is a rare and highly malignant tumor of the urinary system. It is less common for prostate small cell carcinoma coexisting with bladder carcinoma. One such case was reported in this paper. The patient underwent ultrasound-guided transrectal prostate biopsy, and transurethral bladder endoscopy + bladder tumor electric resection. Postoperative pathology and immunohistochemistry showed prostate small cell carcinoma accompanied by high-grade invasive urothelial carcinoma of the bladder and small cell carcinoma. The patient underwent local bladder perfusion chemotherapy, relying on etoposide plus lobaplatin systemic chemotherapy and toripalimab immunotherapy. Prostate MRI, cystoscopy, and prostate-specific antigen (PSA) were performed 12 months after operation. The size of the lesions in the prostate and seminal vesicles had decreased, and there was a significant reduction in PSA levels. Additionally, no masses were detected in the bladder.