This is a prospective, randomized, phase II clinical study in patients with unresectable stage III-IVA esophageal squamous cell carcinoma (ESCC). Eligible patients will be randomly assigned in a 1:1 ratio to two treatment groups. The experimental group will receive 3 cycles of induction therapy with PD-1 antibody plus chemotherapy, followed by radiotherapy, and then maintenance therapy with PD-1 antibody monotherapy. The control group will receive concurrent chemoradiotherapy, followed by maintenance therapy with PD-1 antibody monotherapy. The primary endpoints are the complete response (CR) rate at 3 months after radiotherapy (assessed by investigators) and the 1-year progression-free survival (PFS) rate. Secondary endpoints include overall survival (OS), progression-free survival (PFS), duration of response, objective response rate (ORR), local-regional recurrence-free survival (LRFS), distant metastasis-free survival (DMFS), quality of life, and safety profile.