Background: Immunotherapy has improved outcomes for metastatic colorectal cancer(mCRC) patients with MSI-H/dMMR but has not been effective in MSS/pMMR tumors, which comprise 95% of mCRC. In metastatic cancers, radiotherapy can be used as a local therapy and a sensitizer to PD-1/PD-L1 inhibitors. The synergistic effects of radiotherapy and PD-1/PD-L1 inhibitors can be reinforced by adding cytokines like granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-2(IL-2). Our previous PRaG trial demonstrated that PD-1 inhibitors in combination with radiotherapy and GM-CSF could improve clinical response in patients with advanced refractory solid tumors. We found that mCRC patients with MSS/pMMR could also benefit from the PRaG regimen. Methods: Patients of mCRC with MSS/pMMR were collected from the PRaG trial and PRaG 2.0 trial. A treatment cycle consisted of radiotherapy (5 or 8Gy×2-3f) delivered for one metastatic lesion, PD-1 inhibitor dosing within one week after completion of radiotherapy, GM-CSF 200μg subcutaneous (SC) injection once daily for 14 days(d1-14), or GM-CSF 200μg (d1-7), and then sequentially followed by IL-2 200 million IU SC once daily for 7 days(d8-14). PRaG regimen was repeated every 21 days for at least 2 cycles until no appropriate lesions for irradiation or reached the tolerance dose. Pooled analysis of response rate (ORR), median progression-free survival (PFS), and treatment-related adverse eventswere calculated. Results: With the cutoff date of 31 December 2021, a total of 9 mCRC patients with MSS/pMMR were enrolled. All of the patients completed at least 2 cycles of PRaG therapy and one evaluation. The median follow-up was 7.6 months (95%CI, 3.8, 11.4). The ORR was 22.2%, and the disease control rate(DCR) was 66.7%. The median PFS was 5.6 months (95%CI, 1.5 to 9.7). One patient got complete remission, with PFS over 17months. Treatment-related adverse events (TRAE) occurred in 7 of 9 (77.8%) patients. One patient (11.1%) occurred Grade3 TRAE of renal insufficiency. Conclusions: Our preliminary results suggest that the PRaG regimen could improve clinical outcomes in mCRC patients with MSS/pMMR with acceptable toxicity.