Purple ecchymosis on the buttocks may be related to trauma, injection, mosquito bites and so on. 1. Trauma: If the patient’s buttocks are traumatized and collided, the local tissue injury or capillary rupture and bleeding will lead to local bruising, local swelling, bruising and other symptoms, with the gradual fading of bruising, it may become purple or purple ecchymosis, and it will be accompanied by obvious pain. 2. Injection: If the patient has just had an injection, the speed of medicine input is too fast, and it will bruise at the injection site, which will lead to bruising and pain at the site. 3. idiopathic thrombocytopenic purpura: it is a clinical syndrome of increased platelet destruction due to abnormalities in the immune function, mostly seen in children aged 1-5 years old, with spontaneous bleeding of the skin and mucous membranes as the prominent manifestation, mostly pinpoint-sized intradermal or subcutaneous hemorrhagic dots or bruises and purpura. There are other causes of petechiae on the buttocks, which should be promptly consulted to identify the cause and treated in strict compliance with medical advice.