Anal canal cancer refers to tumors below the dentate line of the rectum to the opening of the anus. Rectal cancer refers to cancer above the dentate line of rectum to the junction of rectum and sigmoid colon, which accounts for about 60-70% of the incidence in colorectal cancer. 1.Blood in stool: accounting for 88%, the location of lesion is low, mainly manifested as blood in stool and change of stool habit. Blood in stool can be fresh blood or dark red blood, which is not mixed with stool. It often appears as mucus blood stool, frequent bowel movement, incomplete defecation and other urgent and heavy feeling, which is easily confused with bacillary dysentery. 2.Change of stool habit: The cancer foci stimulate the intestinal tract and lead to intestinal dysfunction and increase of stool frequency. Change of stool shape. 3.The manifestation of cancer foci invading surrounding organs: when tumor invades sacral plexus nerve, it may cause severe pain; when invading bladder, symptoms such as urinary urgency, painful urination, dyspareunia and hematuria may appear; when women invade vagina and develop vaginal rectal fistula, bloody discharge and feces may appear in vagina. 4.Late stage colorectal cancer may show weakness, weight loss, ascites, cancer fever, etc. When the pathological type is carcinoid tumor, carcinoid syndrome may also appear. It is mainly caused by the production of pentraxin, bradykinin, histamine and catecholamine by carcinoid tumor cells.