There are many reasons for purple patches on the stomach, and it is difficult to diagnose if the dermatologist does not see the picture. Purple patches on the belly may be related to the following factors: 1. Swelling lines: also called stretch marks. It may be due to purple lines caused by growing too fast in the growth process, such as growing too tall or growing too fat too fast causing skin elastic fiber breakage, which is accompanied by a certain amount of itching at this time. This situation does not need special treatment, but if you apply drugs to dispel scar or go to the hospital to do radiofrequency and other treatment, it can reduce the occurrence of purple lines; 2, bleeding disease: due to idiopathic thrombocytopenic purpura or allergic purpura and other cases will cause subcutaneous bruising, can be manifested as purple spots; 3, other skin diseases: such as mosquito bite infection caused by papular urticaria, or inoculation caused by purple spots or erythema.