In men with stomach pain and blood in the urine, the possible causes are: 1. Stones in the urinary system, such as bladder stones or ureteral stones, can cause pain in the small abdomen or lower back pain, which is often severe and accompanied by blood in the eyes. It is recommended that patients with sudden stomach pain and hematuria should undergo ultrasound examination to rule out the presence of urinary stones. 2. Acute infections of the urinary system, such as acute cystitis and urethritis, can cause pain in the small abdomen with hematuria, and can be accompanied by symptoms such as urinary frequency, urinary urgency or painful urination. It is recommended to perform routine urinary system and urine culture examination to further confirm the diagnosis.3. Tumors of the urinary system, such as bladder tumors and ureteral tumors, can cause small abdominal pain with bleeding, which is often more serious and manifests as flesh-eye hematuria throughout. Cystoscopy and CT examination of the urinary system are needed to further rule out the presence of urological tumors.