Dark brown human semen may be hematospermia, the causes of which include acute seminal vesiculitis, prostate cancer, cardiovascular and cerebrovascular diseases. 1. Acute seminal vesiculitis: it is mostly caused by pathogenic microorganisms passing through the urethra and retrograde to the seminal vesicle glands, causing inflammation. During the onset of the disease, many patients will have dark brown semen. 2. Prostate cancer: the cause of prostate cancer is not clear, it may have some relationship with genetic factors, age, obesity and so on. When the disease invades the seminal vesicle glands, the symptom of bloody semen is also easy to appear. 3. Cardiovascular disease: this disease is mostly concentrated in the elderly group, mainly due to the long-term use of antiplatelet drugs, resulting in blood clotting dysfunction, may appear semen is dark brown phenomenon. There are many reasons why semen is dark brown, it is recommended that the patient should not panic too much, should seek medical advice as soon as possible, and follow the doctor’s instructions for treatment.