• Age 18-80, had an Eastern Cooperative Oncology Group performance status score of 0-1;

• Patients had unresectable or recurrent esophageal squamous cell carcinoma that precluded esophagectomy or definitive chemoradiation, or distant metastatic disease;

• Patients had received no previous systemic therapy (patients who had progressed ≥6 months after \[neo\]adjuvant therapy or definitive chemoradiation were eligible);

• Patients accepted at least one cycle of anti-PD-1 immunotherapy combined chemotherapy as 1st line treatment.

• All lesions of current diagnosis did not receive local treatment such as radiotherapy, surgery, radiofrequency ablation before enrollment.

• The measurable lesion was determined by the investigator based on the RECIST 1.1 assessment. A lesion located in a previous radiotherapy area can be considered a target lesion if it is confirmed to progress and is considered to be measurable according to RECIST 1.1.

• Estimated survival time \>12 weeks.

• The function of vital organs meets the following requirements:

• Neutrophil absolute count (ANC) ≥ 1.5 × 10\^9 / L platelets ≥ 100 × 10\^9 / L; Hemoglobin ≥ 9g / dL; serum albumin ≥ 2.8g / dL; Total bilirubin ≤ 1.5 × ULN, ALT, AST and / or AKP ≤ 2.5 × ULN; if there is liver metastasis, ALT and / or AST ≤ 5 × ULN; if there is liver metastasis or bone metastasis AKP ≤ 5 × ULN; serum creatinine ≤ 1.5 × ULN or creatinine clearance \> 60 mL / min; For patients with pulmonary lesions or previous lung irradiation who are known or suspected to have impaired lung function, the forced expiratory volume (FEV1) for 1 second of lung function must be above 1L.

• Female subjects of childbearing age must have a negative urine or serum pregnancy test within 72 hours prior to randomization. Subjects agreed to adequate contraception during the trial.

⁃ The patient is voluntarily enrolled and obtained the informed consent form signed by the patient or his legal representative.