Objective:To analyze the effectiveness and safety of the second course radiotherapy for unresectable colorectal cancer liver metastases.Methods:We retrospectively collected the data of 28 patients with unresectable colorectal cancer liver metastases who received the second course radiotherapy at Peking University Cancer Hospital and Institute from 2017 to 2023, to analyze the feasibility of re-irradiation.Results:For the 28 patients, the median follow-up time after re-irradiation was 20.2 months. The median time interval between the first- and second-course radiotherapy was 11.1 months. The median biologically effective doses of the first- and second-course radiotherapy were 100 Gy and 96 Gy, respectively. Stereotactic body radiotherapy was administered to 25 patients (89.3%) during the first course and 24 patients (85.7%) during the second course of radiotherapy. The mean equivalent dose in 2 Gy fractions to the normal liver was 10.1 Gy in the first-course radiotherapy and 7.9 Gy in the second-course radiotherapy. The complete response rate, partial response rate, and objective response rate after re-irradiation were 54.5%, 18.2%, and 72.7%, respectively. After re-irradiation, the 2-year cumulative local failure rate was 17.0% when calculated based on patients and 15.1% when calculated based on lesions, the 1-year progression-free survival rate was 27.4%, and the 3-year overall survival rate was 46.7%. The second-course radiotherapy was well tolerated, with most patients (75.0%) experiencing grade 1-2 acute adverse reactions and only one case (3.6%) experiencing grade 3 acute adverse events.Conclusions:Second course radiotherapy is an effective and safe treatment approach for selected patients with unresectable colorectal cancer liver metastases.