Blood in the urine after sex may be due to urethral injury, urinary tract infection, urinary tract stones, tumors and other reasons. 1. Urethral injury: the female urethra is adjacent to the vaginal opening, and during sex, the action is too violent, the male penis is too large, the female vagina is too tight, and the sex position is abnormal, leading to the female urethral opening to be pulled, torn, expanded, and so on, and there is a urethral injury, bleeding, and so there is blood in the urine after sex. And male sexual life may also be due to the glans pressure swelling and blood in the urine. 2. Urinary tract infection: before sex, both men and women did not carry out the vulva and urethra area of cleanliness and hygiene, sexual life environment is not hygienic, multiple sexual partners, frequent sexual intercourse, one party suffers from sexually transmitted diseases and other factors lead to sexual life during the process of bacterial, viral, syphilitic spirochetes, mycoplasma and other pathogens through the urethra retrograde infection, the emergence of urinary tract infection, so there is blood in the urine. 3. Urinary stones: Sex may stimulate the stones in the urethra and bladder, resulting in increased friction between the stones and the mucosa of the urinary tract, causing mucosal damage and bleeding. 4. Tumor: bladder cancer, penile cancer and other tumors are stimulated during sex and bleeding of tumor tissue occurs. Blood in urine after sex may be related to many factors, so it is recommended to consult doctor for clear diagnosis.