Colonoscopy has diagnostic value for colorectal cancer. Through colonoscopy, we can directly observe the changes of intestinal wall and lumen of the whole colon, and determine the location, size and infiltration range of the tumor, and biopsy can confirm the diagnosis. Colonoscopy biopsy is only a method to confirm the diagnosis of colon cancer, but it is not necessary to take biopsy for cancer, some benign diseases such as intestinal polyps and inflammatory bowel disease also need to take biopsy. Colorectal cancer includes colon cancer and rectal cancer, which are common malignant tumors. Colorectal cancer is relatively insidious in origin, and only positive fecal occult blood is seen in the early stage. Clinical symptoms appear later. Changes in bowel habits and stool characteristics are often the earliest symptoms of the disease. Most of them are mainly bloody stools or pus-blood stools with urgency. Sometimes constipation may also be persistent. The stool becomes fine in nature. It may also present as diarrhea with pasty stools, alternating between diarrhea and constipation. Abdominal pain is also an early symptom, usually a dull pain in the right abdomen, or involving both the right upper abdomen and the middle and upper abdomen. Or postprandial abdominal pain. Abdominal masses, rectal masses, etc. Systemic condition may include anemia, low fever, progressive wasting, cachexia, ascites, etc. The key to the treatment of colorectal cancer is early detection and early diagnosis, so that there is a chance of eradication. The only curative method for colorectal cancer is early resection of the cancer. The prognosis of this disease also depends on early diagnosis and surgical eradication.