An 81-year-old male patient presented to the department of gastroenterology with increasing lower abdominal pain for 2 years, aggravated with bloody stool for 1 month. Computed tomographic examination revealed a huge cyst (207 × 93 × 208 mm3) in the abdominal cavity, absence of bladder, thickening and strengthening of the rectal wall, and benign prostatic hyperplasia. Colonoscopy showed colon cancer and surgery was planned. Interestingly, after magnetic resonance imaging and cystography, we found colon cancer and a large bladder diverticulum rather than tumor metastasis or others. Severe bacteremia occurred in the elderly chronic obstructive pulmonary disease patient before operation. After careful consideration, we decided to take a large risk and combined urology and gastrointestinal surgery professionals to carry out bladder diverticulectomy, cystostomy, radical resection of rectal carcinoma, and so on. Fortunately, the patient recovered well after the operation. In addition to the common tumor metastasis and cystadenoma, the abdominal mass should also be alert to the rare bladder diverticulum, which eventually leads to diagnostic confusion. Multidisciplinary diagnosis and treatment has become an important treatment for complex diseases.