There are many reasons why the colonoscope cannot pass, which can be benign lesions or malignant tumors, mainly including the following: 1. Malignant tumors, which can be occupying lesions or malignant ulcers. The occupancy will lead to narrowing of the intestinal lumen, so the body of the colonoscope cannot pass through. And ulcers will make the intestinal wall thicken, which will also cause stenosis, in addition to bleeding, thinning of the intestinal wall, it is possible that the mirror body through, will cause bleeding aggravated, or even perforation; 2, benign lesions, benign lesions may also lead to the narrowing of the intestinal lumen, such as some polyps, and ulcers, some benign ulcers are also very serious, if forced through, will also lead to bleeding, or even perforation of the situation; 3. In addition, lesions outside the intestinal tract compress the intestinal canal and cause narrowing of the intestinal lumen, which may also lead to the inability to pass the mirror body.