How to detect colorectal cancer at an early stage is the key to treatment of colorectal cancer. Generally speaking, the following conditions should be paid special attention, 1. Blood in stool: the time and nature of blood in stool differ from different parts of the colon. Blood in stool is often the first symptom of rectal cancer patients, the amount is small in the early stage, and blood marks appear on one side of the stool strip; blood in stool of sigmoid colon cancer is similar to rectal cancer, but the color of stool will become dull, or be sauce purple or black purple. 2. Change in stool habit: including change in stool time and frequency, as well as constipation and unexplained diarrhea. 3. Change of stool shape: the stool will be compressed by the cancer, and the stool will become thinner or have indentations, sometimes with a trace of blood. 4. Diarrhea: Some patients have diarrhea as the first symptom, which may be mucus-blood stool, mucus-purulent stool or loose and thin stool, etc. 5. Painful defecation: about 50% of rectal cancer patients have pain when defecating. 6. Abdominal pain: some patients have abdominal pain as the first symptom, while others with incomplete intestinal obstruction have paroxysmal colic and may be accompanied by abdominal distension. 7. Anemia and weakness