Short-course radiotherapy combined with immunotherapy may bring revolutionary changes to the total neoadjuvant therapy mode for locally advanced ultra low rectal cancer to preserve the organs. In view of the shortcomings of the current otal neoadjuvant therapy model for locally advanced ultra low rectal cancer, we will explore the feasibility of a new model of short-course radiotherapy combined with immunotherapy, and develop a possible optimal plan based on the existing theoretical basis, namely "short-course radiotherapy + PD-L1 monoclonal antibody combined with CAPEOX chemotherapy for 8 cycles", and explore the efficacy and adverse effects of this model. The study will also attempt to explore the characteristics of the treatment beneficiary population, explore the characteristics of the treatment beneficiary population by multi-dimensional tumor and microenvironmental information through multi-omics sequencing analysis, attempt to build an efficacy prediction model, early screening of the treatment beneficiary population for precise treatment, and thus explore a new model of radiotherapy combined with immunotherapy for the poplation who can be achieved organ preservation.

Inclusion Criteria:

1. Patients who are willing to receive neoadjuvant therapy.
2. ≧18 years old.
3. Diagnosed by digital rectal examination, colonoscopy, and high-resolution MRI of the pelvis, the tumor is less than or equal to 5 cm from the anus.
4. Histologically diagnosed as rectal adenocarcinoma.
5. The clinical staging by pelvic contrast-enhanced CT and pelvic high-resolution MRI were stage I, II and III.
6. MMR protein detection or MSI gene detection of rectal cancer specimens confirmed pMMR or MSS before treatment .
7. The patient is difficult cured by anal reserve procedure based on the primary physician's practice.
8. The patient has good compliance and can come to the hospital for re-examination as required.
9. ECOG Scale of Performance Status score 0-1 point.
10. Have not received anti-tumor and immunotherapy before enrollment.
11. Laboratory inspections must meet the following standards:

1) White blood cell count>3.5×109/L, absolute value of neutrophils>1.8×109/L, platelet count ≥75×109/L, hemoglobin ≥100g/L; 2) INR≤1.5, and APTT≤1.5 times the upper limit of normal or partial prothrombin time (PT) ≤1.5 times the upper limit of normal; 3) Total bilirubin ≤ 1.25 times the upper limit of normal; ALT and AST < 5 times the upper limit of normal; 4) 24h creatinine clearance >50mL/min or serum creatinine <1.5 times the upper limit of normal.

12. Voluntarily participate in this study and sign the informed consent.

Exclusion Criteria:

1. History of other malignant diseases in the past 5 years.
2. Patients with metastases from other sites (stage IV patients).
3. Patients with positive lateral lymph nodes by pelvic contrast-enhanced CT and pelvic high-resolution MRI.
4. Patients with intestinal obstruction, intestinal perforation, intestinal bleeding, etc. requiring emergency surgery.
5. Known allergic to oxaliplatin, capecitabine, PD-L1 monoclonal antibody and other drugs.
6. Pathologically suggested signet ring cell carcinoma and mucinous adenocarcinoma.
7. dMMR or MSI-H patients.
8. The patient is accompanied by any unstable systemic disease, including but not limited to: severe infection, uncontrolled diabetes, hypertension uncontrolled by medication, unstable angina, cerebrovascular accident or transient cerebral ischemia, myocardial Infarction, congestive heart failure, severe cardiac arrhythmia requiring medication, hepatic, renal or metabolic disease; disease affecting the patient's life.
9. The disease (such as mental illness, etc.) or condition (such as alcoholism or drug abuse, etc.) associated with the patient will increase the risk of the patient receiving the trial drug treatment or affect the patient's compliance with the trial requirements, or may confuse the research results.
10. Active autoimmune disease that may worsen while receiving immunostimulants.
11. Known history of positive HIV test or known acquired immunodeficiency syndrome.
12. Patients who are using immunosuppressive agents, except for the following conditions:

1) Intranasal, inhaled, topical steroids, or topical steroid injections (eg, intra-articular injections); 2) Physiological doses of systemic corticosteroids ≤10 mg/day prednisone or equivalent; 3) Steroids used to prevent allergic reactions (eg, before CT scan). 13. Received any other experimental drug treatment or participated in another interventional clinical trial within 30 days before screening 14. Women who are pregnant or breastfeeding or who plan to become pregnant or breastfeeding during the study period; men or women who are unwilling to take effective contraceptive measures.

15. Vulnerable groups, including mentally ill, cognitively impaired, critically ill patients, minors, etc.

16. Other conditions that the investigator judges that the patient is not suitable to participate in the clinical study, etc.