Rectal cancer and colon cancer are collectively known as colorectal cancer, which is one of the most common malignant tumors. Among gastrointestinal tumors, colorectal cancer ranks third after stomach cancer and esophageal cancer. The age of onset is mostly above 40 years old, and there are more men than women. The occurrence of colorectal cancer is related to chronic inflammation (ulcerative colitis, schistosomiasis japonica), colorectal polyps, adenoma and certain genetic factors. In addition, people who often eat high protein and high fat have more anaerobic bacteria in the intestine, which can break down bile acids into unsaturated polycyclic hydrocarbons, which is a carcinogenic substance, coupled with low intake of fiber-based foods and long fecal storage, carcinogenic substances gather, which can easily lead to colorectal cancer. Early detection of colorectal cancer should pay special attention to the following aspects: 1. Blood in stool is one of the early symptoms of all colorectal cancers, but the time and nature of colorectal cancers of different parts are different. Blood in stool is often the first symptom of rectal cancer patients, and the amount is very small in the early stage, mostly with fresh blood marks on one side of the stool strip. In a few patients, after the stool is discharged, a larger amount of fresh blood in the form of drops is discharged; in sigmoid colon cancer, because it is closely connected to the rectum, the characteristics of blood in stool of sigmoid colon cancer are similar to those of rectal cancer. The first symptom is not always the patient’s first symptom. Sometimes, due to the small amount of blood or the long stay in the body, the blood cannot be detected by the naked eye, but the fecal occult blood test is positive. Blood in stool of colorectal cancer should be distinguished from blood in stool caused by hemorrhoids, anal fissure, bacillary dysentery, enteritis, intestinal polyps, ulcer perforation and other diseases. Change in stool habit includes change in stool time and frequency, as well as constipation or unexplained diarrhea. Patients with rectal cancer may have more stools, but not much stool each time, or even no stool at all, just some mucus and blood, and the feeling of incomplete defecation. Some patients may have diarrhea after constipation, or only dry stool at the beginning and thinning at the end, or repeatedly alternating constipation and diarrhea. If the cancer protrudes in the rectal cavity and compresses the stool, the stool strips will often become thinner and the shape can be changed to flat. 4, diarrhea Some patients can have diarrhea as the first symptom. Patients have more bowel movements per day, which can be mucus and blood, mucus and thick blood or loose and thin stools, accompanied by a feeling of urgency, which should be distinguished from bacillary dysentery. 5.Painful defecation About 50% of rectal cancer patients have painful defecation, the degree of which may be mild or severe. 6.Abdominal pain Some patients have vague abdominal pain as the first or prominent symptom, while others show typical abdominal pain of incomplete intestinal obstruction, i.e. the pain is paroxysmal colic and accompanied by abdominal distension.