Malignant melanoma of the anorectum was first discovered and reported by Moore in 1857 and is relatively rare and has a very poor prognosis. After the skin and the eye, the third most common site for malignant melanoma of the anorectum, about 70-90% of them occur below the dentate line of the anal canal or at the anal verge, covered by squamous epithelium. Here, a large number of melanocytes accumulate. Melanocytes or their parent cells originate from ectodermal neural crest cells and migrate to the skin, ocular mucosal surfaces and the nervous system during embryonic development. A variety of factors such as endocrine metabolic disorders, chemical irritation, or high-energy radiation damage can cause melanocyte malignancy. Some studies suggest that malignant melanoma of the anal canal may be primary. From an embryologic developmental point of view, it is impossible for the small intestine and colorectal mucosa, which originate from the endoderm, to develop such lesions. Most tumors can produce melanin, a few can have no melanin, and only 20% of cases have significant melanin. Melanoma is more prone to metastasis. The main metastasis is hematogenous metastasis, which occurs early, mainly to liver, lung, brain and bone; secondly, lymphatic metastasis, which occurs early in inguinal lymph nodes, closed-hole lymph nodes, and lymph nodes adjacent to abdominal aorta; again, direct infiltration, where the tumor invades pelvic tissues, and usually not often invades adjacent organs such as uterus and bladder. Clinical stage: Stage I cancer is limited without local infiltration; Stage II cancer has local infiltration but no distant metastasis; Stage III cancer has distant metastasis. The clinical manifestations of melanoma are commonly as follows: 1. dark red masses prolapse from the anus, similar to thrombosed hemorrhoid impaction; 2. blood in the stool, similar to hemorrhoid bleeding due to fecal abrasion or trauma to the tumor. Generally, it is mostly fresh blood, sometimes it is also mucus blood stool or dark brown overflow, accompanied by foul odor; 3, rectal and anal canal irritation symptoms, similar to hemorrhoid attack, urgency, sometimes change in defecation habit, often have a sense of incomplete defecation, sometimes alternating diarrhea and constipation; 4, anal pain, mostly because the tumor has invaded the anal sphincter; 5, local protruding mass, with short and wide tip, or nodular, sometimes cauliflower-shaped, mostly purple-black or brown. Most of them are purple-black or brown. Because of the insidious location of the disease and the lack of specific symptoms, it is easy to miss and misdiagnose, and the initial diagnosis rate is very low, with a misdiagnosis rate of 87% reported in the literature. It is often misdiagnosed as prolapsed hemorrhoids, thrombosed external hemorrhoids, bleeding necrosis of anal canal polyps, colitis or adenocarcinoma of the rectum. In particular, anaplastic malignant melanoma is more likely to be misdiagnosed because of its rarity. Therefore, we remind our readers that once the above symptoms appear, do not be careless and do not go to non-professional clinics to be satisfied with the diagnosis of prolapsed hemorrhoids, thrombosed external hemorrhoids, bleeding necrosis of anal canal polyps, colitis, etc. Always go to a regular hospital to be treated by a professional doctor and get recovered.