In this open-label phase II study, patients will be scheduled for neoadjuvant treatment with PD-1 antibody following preoperative Chemoradiotherapy with capecitabine for pMMR/MSS rectal cancer staged as locally advanced (cT3-T4N+/-M0 for rectal cancer). This treatment will be given during the window period until surgical resection of the tumor.

Inclusion Criteria:

• Patients who personally provided written consent for participation in the study
• Treatment-naive patients with rectal cancer, in whom the inferior margin of the tumor was at a distance of 12 cm or less from the AV before CRT
• Primary rectal cancer histopathologically confirmed to be adenocarcinoma
• Clinical stage of T3,and T4 ,N any,M0,before CRT
• Macroscopic radical resection could be feasible, based on diagnostic imaging before CRT
• Patients with the ECOG performance status of 0 or 1 at the time of enrollment
• Patients without distant metastasis on the imaging test before CRT
• Preoperative biopsy of tumor immunohistochemistry TPS>1% or CPS>1
• Life expectancy of greater than 2 years
• No signs of intestinal obstruction; or the obstruction has been relieved after the proximal colostomy operation
• Hematology: WBC>4000/mm3; PLT>100000/mm3; Hb>10g/dL
• Liver function: SGOT and SGPT are less than 1.5 times the normal value; bilirubin is less than 1.5mg/dL
• Renal function: creatinine <1.8mg/dL Others: non-pregnant or breast-feeding women; no other malignant diseases (except for non-melanoma or cervical carcinoma in situ) within 5 years or during the same period; no mental illness that causes the inability to obtain informed consent; no other serious diseases that can shorten the survival time disease.
• Have not received rectal surgery in the past;
• Have not received chemotherapy or radiotherapy in the past;
• Have not received biological treatment in the past;
• Past endocrine therapy: unlimited.

Exclusion Criteria:

• Rectal cancer with unstable microsatellite (MSI or dMMR);
• Preoperative biopsy of tumor immunohistochemistry TPS≤1% or CPS≤1
• Known history of human immunodeficiency virus (HIV) or chronic hepatitis B or C (high copy viral DNA);
• Autoimmune diseases;
• Other active clinical serious infections (>NCI-CTC version 3.0);
• Patients in clinical phase I;
• There is evidence that there is distant metastasis before surgery;
• Cachexia, decompensation of organ function;
• Have a history of pelvic or abdominal radiotherapy;
• Multiple primary cancers;
• Patients who need treatment for seizures (such as steroids or anti-epileptic treatment);
• Have a known additional malignant tumors within 5 years. Exceptions include basal cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer;
• Chronic inflammatory bowel disease, intestinal obstruction;
• Drug abuse and medical, psychological or social conditions may interfere with patients' participation in research or have an impact on the evaluation of research results;
• Known or suspected to be allergic to the study drug or to any drug given in connection with this test;
• Any unstable conditions or situations that may endanger patient safety and compliance