Inclusion Criteria:  Histologically confirmed ESCC; Patients have not received neoadjuvant therapy. Thoracic ESCC with R0 resection and positive postoperative lymph nodes; Have a performance status of 0 or 1 on the ECOG Performance Scale; Age 18-70 years old, both men and women; Be willing and able to provide written informed consent/assent for the trial; Demonstrate adequate organ function , all screening labs should be performed within 10 days of treatment initiation; Female subject of childbearing potential should have a negative urine or serum pregnancy within 72 hours prior to receiving the first dose of study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required; Be willing to provide postoperative tissue specimens. After tumor recurrence, continue to be willing to provide tissue samples such as needle biopsy.  Exclusion Criteria:  Neoadjuvant therapy (radiotherapy, immunotherapy, or chemotherapy) for ESCC; Non-R0 resection for ESCC; Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 7 days prior to the first dose of study drug; Has active autoimmune disease that has required systemic treatment in the past 2 years (i.e. with use of disease modifying agents, corticosteroids or immunosuppressive drugs); Has severe hypersensitivity and adverse events (≥Grade 3) to pembrolizumab and/or any PD-1/PD-L1 inhibitors.