• Patients who provided written informed consent form for participation in this study;

• Aged 18-75 years old, male or female;

• Histologically confirmed pathological diagnosis of proficient mismatch repair/microsatellite stable(pMMR/MSS) rectal adenocarcinoma;

• The lower margin of the tumor is ≤10cm from the anal verge;

• Clinical Stage (according to the 8th edition of AJCC) T3NanyM0 and confirmed on imaging to fulfil at least any of the following: （1）MRF (+)，（2）EMVI（+），（3）LPLN（+）；or T4NanyM0 with or without one of the above three;

• Those who are expected to achieve R0 resection;

• Able to swallow tablets normally;

• Patients with the ECOG performance status of 0 or 1 at the time of enrollment;

• Patients have not received any previous anti-tumor therapy for rectal cancer;

⁃ Planning to undergo surgery after completion of neoadjuvant therapy;

⁃ Have no contraindications to surgery;

⁃ Normal function of major organs, including:

∙ Routine blood tests (no blood components, cell growth factors, leukocyte boosters, platelet boosters, or anaemia-correcting drugs will be allowed within 14 days prior to the first dose of study drug):White blood cell count ≥ 4.0 x 109/L; Neutrophil count ≥ 1.5 x 109/L; Platelet count ≥100×109/L; Hemoglobin ≥90 g/L

‣ Blood biochemistry: Total bilirubin ≤ 1.5 x ULN; ALT ≤ 2.5×ULN, AST ≤ 2.5×ULN; Serum creatinine ≤ 1.5 x ULN, or creatinine clearance ≥ 50 mL/min (Cocheroft-Gault formula)

‣ Coagulation: International normalised ratio (INR) ≤ 1.5 x ULN; Activated partial thromboplastin time (APTT) ≤ 1.5×ULN

⁃ Female subjects of childbearing potential are required to have a negative serum pregnancy test within 72 hours prior to initiation of study drug administration and to use effective contraception (e.g., intrauterine device, birth control pills, or condoms) during the trial period and for at least 3 months after the last dose of study drug; for male subjects whose partner is a female of childbearing potential, effective contraception should be used during the trial period and for 3 months after the last dose of study drug; and for male subjects whose partner is a female of childbearing potential, effective contraception should be used during the trial period and for 3 months after the last dose of study drug. Use effective contraception;