15-year-old boy seminal emission is bright red may be caused by acute injury, prostatitis, seminal vesiculitis and other factors. 1. Acute injury: boys in some strenuous exercise, the lower body is damaged, resulting in damage to the reproductive tract local bleeding, in the spermatozoa, semen mixed with blood, it will lead to spermatozoa color is red. 2. Prostatitis: Prostatitis may be induced by holding urine for a long time, sitting for a long time and other factors. When the infection is serious, it may lead to semen with red color, which may be accompanied by perineal distension and pain, frequent urination, urgency of urination and other symptoms. 3. Seminal vesiculitis: it may be related to the infection of neighboring organs, due to the infection, resulting in the local tissue of the seminal vesicles is damaged and bleeding, which may cause hematospermia, and in the case of spermatozoa, there may also be mild pain. The 15-year-old boy’s spermatozoa is bright red, indicating the existence of abnormalities, it is recommended that timely medical examination and treatment, to avoid the occurrence of other diseases.