The goal of this study is to compare the postoperative anal function of patients with ultra-low rectal cancer after Parks operation (colon anal anastomosis) and Bacon operation (colon anal pull-out anastomosis), which may provide clinical evidence for the improvement of anal function and quality of life.

The main questions it aims to answer are: the difference of anal function 1 year after surgery type of study: clinical trial participant population: patients with low rectal cancer Participants will receive Parks operation of Bacon operation If there is a comparison group: Researchers will compare Parks and Bacon operation to see if the anal function 1 year after surgery is different.

objective: To compare the postoperative anal function of patients with ultra-low rectal cancer after Parks operation (colon anal anastomosis) and Bacon operation (colon anal pull-out anastomosis), which may provide clinical evidence for the improvement of anal function and quality of life.

primary outcome: Low Anterior Resection Syndrome (LARS) score 1 year after surgery secondary outcomes: 1. LARS score at 3 months after surgery 2. LARS score at 6 months after surgery 3. Postoperative Quality of Life Score 4. The incidence of postoperative anastomotic complications

Inclusion Criteria:

1. Men or women, aged between 18 and 75 years;
2. Pathological diagnosis of rectal cancer before operation, with the distance from the lower edge of the tumor to the dentate line ≤ 3cm by MR imaging;
3. No local complications (including complete/ incomplete obstruction, active bleeding, local invasion of sphincter or levator ani, etc.) before operation;
4. Suitable for anal preservation surgery discussed by MDT;
5. Adequate bone marrow, liver, renal and cardiac function meeting the requirements of surgery and anesthesia;
6. R0 resection is expected technically;
7. Provision of written informed consent.

Exclusion Criteria:

1. Previous history of malignant colorectal tumors;
2. Complications such as obstruction and gastrointestinal bleeding that need an emergency operation;
3. Unachievable R0 resection due to invasion of adjacent organs by primary tumor;
4. Multiple primary tumors;
5. History of other malignancy;
6. Participation in other clinical trials within the previous 4 weeks of enrollment;
7. ASA physical status score ≥ IV level and/or ECOG performance status ≥ 2;
8. Intolerance of surgery due to liver, renal, cardiopulmonary or coagulation dysfunction, or underlying diseases;
9. History of serious mental disorders;
10. Women in pregnancy or lactation period;
11. Uncontrolled infection before operation;
12. Unsuitable to be included due to other clinical and laboratory conditions by the judgement of investigators.