Rectal cancer is one of the most common gastrointestinal cancers, and its epidemiology is characterized by a significant geographic distribution, a consistently increasing incidence and mortality rate, an increasing proportion of women, an aging age of onset, and genetic and dietary factors that are particularly prominent in its development. Unlike many other malignant diseases, rectal cancer has the opportunity to be detected, diagnosed and thoroughly treated in the early stages of the disease. Early surgery for rectal cancer has good healing and long survival. Advantages of ultra-early detection of rectal cancer 1. filling the gap of non-invasive detection of rectal cancer 2. filling the gap of suitable technology at the bedside of rectal cancer 3. using autologous cellular exudate to achieve qualitative detection 4. breaking the technical bottleneck of large area census and centralized clinical screening of rectal cancer 5. increasing the effective means of simple and easy follow-up process of post-operative patients of rectal cancer Applicable population People over 40 years old, rectal cancer patients or post-operative patients recovery period, etc.