Thinning stools and normal colonoscopy may be caused by anal sphincter dysfunction, intestinal flora imbalance, constipation and other reasons. 1. Anal sphincter dysfunction: if the stool becomes thin, but the colonoscopy is normal, it may be caused by anal sphincter dysfunction, anal sphincter dysfunction patients may also appear defecation obstacles, constipation, defecation effort and so on. 2. Intestinal flora dysbiosis: intestinal flora dysbiosis can also lead to thin stools or even unformed stools, when the condition is relatively mild, colonoscopy is not easy to find abnormalities. 3. Constipation: Constipation can also cause thinning of stools, and serious patients may even have defecation disorder or difficulty in defecation, and when constipation is more serious, forced defecation may also lead to anal tearing. It is recommended that patients go to the regular hospital in time, to clarify the cause of the disease, under the guidance of professional doctors for treatment.