(1) Background: At present, the efficacy and safety of thoracic radiotherapy (TRT) after chemo-immunotherapy (CT-IT) in patients with extensive-stage small-cell lung cancer (ES-SCLC) still remain unclear. The purpose of this study was to evaluate the role of TRT after CT-IT in patients with ES-SCLC. (2) Methods: From January 2020 to October 2021, patients with ES-SCLC treated with first-line anti-PD-L1 antibody plus platinum-etoposide chemotherapy were enrolled retrospectively. The survival data and adverse events data of patients treated with or without TRT after CT-IT were collected for analysis. (3) Results: A total of 118 patients with ES-SCLC treated with first-line CT-IT were retrospectively enrolled, with 45 patients with TRT and 73 patients without TRT after CT-IT. The median PFS and OS in the CT-IT + TRT group and CT-IT only group were 8.0 months versus 5.9 months (HR = 0.64,p= 0.025) and 22.7 months versus 14.7 months (HR = 0.52,p= 0.015), respectively. The median PFS and OS in all 118 patients treated with first-line CT-IT were 7.2 and 19.8 months with an ORR of 72.0%. In multivariate analyses, liver metastasis and response to CT-IT were shown to be independent prognostic factors of PFS (p< 0.05), while liver metastasis and bone metastasis were independent predictive factors of OS (p< 0.05). Although TRT was significantly associated with better PFS and OS in univariate analysis, the association of TRT and OS failed to reach statistical significance (HR = 0.564,p= 0.052) in multivariate analysis. There was no significant difference in adverse events (AEs) between two treatment groups (p= 0.58). (4) Conclusions: ES-SCLC patients treated with TRT after first-line CT-IT had prolonged PFS and OS with an acceptable safety profile. Further prospective randomized studies are necessary to explore the efficacy and safety of this treatment modality for ES-SCLC in future.