Dark red semen indicates the presence of hematospermia, which may be caused by trauma, infectious factors, tumors and other factors. 1. Trauma or surgery: excessive force during sex, testicular damage or pelvic fracture may cause damage to the sexual organs, leading to local bleeding, which will cause blood in the semen, resulting in hematospermia. 2. Infectious factors: for example, seminal vesiculitis, seminal vesicle tuberculosis and other lesions may lead to local tissue damage in the seminal vesicles, resulting in blood in the semen, and then hematospermia, which may be accompanied by ejaculatory pain and other symptoms, and in severe cases, can cause male infertility. 3. Tumor: For example, prostate cancer, after the aggravation of the lesion, the local tissue necrosis, bleeding, it is easy to cause hematospermia, which may be accompanied by perineal distension and pain, blood in the urine, erectile dysfunction, dysuria, urination, frequent urination and other symptoms. Dark red semen indicates the presence of abnormalities, and men are advised to seek medical examination and treatment as soon as possible to avoid delays.