There are many reasons for pain on the left side of the anus: 1. It may be an anal gland infection that spreads along the anal canal to the left perianal skin, resulting in a perianal abscess, which is positive to touch and painful with fluctuating sensation. 2. It may be an anal fissure, such as a dry stool that can easily damage the skin of the anal canal, resulting in skin fissures and painful fissures after forceful defecation or a large diet of stimulating food or alcohol. The reason for this is the painful fissure. 3, there may be mixed hemorrhoids, such as mixed hemorrhoids, if the hemorrhoids out of the anus can not be returned, which leads to hemorrhoid vein tortuous, swelling, the formation of thrombosis, and then embedded necrosis, will cause pain on the left side of the anus. This situation should be promptly treated surgically, such as the external peeling and internal ligation of mixed hemorrhoids.