This is an open-label, multi-center, single-arm clinical study. All patients received concurrent chemoradiation therapy (CRT) followed by 4 cycles of tislelizumab combined with CAPOX, then underwent clinical response assessment. Patients who achieved CR (cCR+ pCR confirmed by local resection of ncCR) continue tislelizumab combined with CAPOX for another 4 cycles and tislelizumab for 9 cycles, then Watch and Wait. Patients who did not achieved CR underwent total mesorectal excision (TME).

Inclusion Criteria:

1. Able to provide written informed consent, understand and comply with the requirements and evaluation schedule
2. ≥18, ≤75 years old
3. Histologically confirmed rectal adenocarcinoma
4. immunohistochemistry confirmed pMMR (positive for MLH1, MSH2, MSH6 and PMS2), or PCR /NGS confirmed MSI-L or MSS
5. The distance from the lower edge of the tumor to the anal verge is ≤5 cm through colonoscopy, digital anal examination or MRI
6. clinical stage cT1-3N1M0 or cT2-3N0M0 (the 8th UICC/AJCC; T and N is evaluated by MRI)
7. Resectable primary tumor assessed by the Investigator
8. Have not received any anti-tumor treatment for rectal cancer
9. ECOG PS ≤ 1
10. Adequate organ function
11. Female subjects with the ability to become pregnant must have a serum pregnancy test with a negative result within 72 hours before the first dose, and be willing to use highly effective contraceptive methods during the trial and 120 days after the last dose. Male subjects whose partners are women of childbearing potential should be surgically sterilized or agree to use a highly effective method of contraception during the trial and for 120 days after the last dose.

Exclusion Criteria:

1. Histologically confirmed poorly differentiated/undifferentiated adenocarcinoma, mucinous adenocarcinoma and signet ring cell carcinoma
2. Have received any treatments for rectal cancer, or evidence of distant metastasis
3. Presence of following high risk factors assessed by MRI: MRF +, EMVI+, cN2, Positive lateral lymph nodes, T3d
4. Presence or in high risk of obstruction, perforation or bleeding;
5. Not suitable for long-course radiotherapy
6. Cannot tolerate surgery
7. ≥2 colorectal cancer lesions at the same time
8. Contraindications for MRI examination
9. Other malignant tumors in the past or at the same time
10. Have an active autoimmune disease requiring systemic therapy within the past 2 years
11. HIV infection
12. Untreated chronic hepatitis B or chronic hepatitis B virus (HBV) carriers (HBV DNA > 500 IU/mL) or active HCV carriers with detectable HCV RNA;
13. Hypersensitivity to any ingredient of tislelizumab, capecitabine, and oxaliplatin or to any component of the container
14. Other conditions judged by the researcher that do not meet the enrollment requirements