The most common cause of fine stool after rectal cancer surgery is caused by anastomotic stenosis, which is a normal phenomenon, but it may also be caused by recurrence of rectal cancer, in which case vigilance is needed. 1. Anastomotic stenosis: postoperative anastomotic stenosis of rectal cancer can lead to fine stool. There are many reasons leading to anastomotic stenosis, which may be caused by edema or anastomotic scar hyperplasia, which are all normal phenomena. The stenosis caused by edema only needs to wait for the edema to subside, while the stenosis caused by scarring may need dilatation treatment. 2. Recurrence of rectal cancer: recurrence of rectal cancer may also lead to localized stenosis, which makes the stool thinner, and this situation may need another operation for treatment. If the symptom of thin stool appears after rectal cancer surgery, one should consult doctor in time to find out the cause and give treatment to avoid delaying the condition.