Objective To explore the values of peripheral blood inflammatory parameters in predicting the efficacy of immunotherapy and prognosis after immunotherapy in patients with proficient mismatch repair(pMMR)metastatic colorectal cancer(mCRC).Methods The clinical data of 44 inoperable pMMR mCRC patients who received immunotherapy in Zhongshan Hospital(Xiamen Branch),Fudan University from February 2019 to February 2024 were analyzed retrospectively.The pre-treatment peripheral blood inflammatory parameters such as neutrophil-to-lymphocyte ratio(NLR),platelet-to-lymphocyte ratio(PLR),pan-immune-inflammation value(PIV)and systemic immune-inflammation index(SⅡ)were collected.The receiver operating characteristic(ROC)curves were drawn to evaluate the predictive value of inflammatory parameters on the efficacy of immunotherapy.The effects of inflammatory parameters on the prognosis after immunotherapy were evaluated according to the optimal cutoff value obtained by ROC curves.Univariate and multivariate Cox proportional hazard models were used to analyze the risk factors affecting the prognosis of patients with pMMR mCRC.Results NLR,PLR,PIV and SⅡ had some predictive values on the efficacy of immunotherapy in inoperable pMMR mCRC patients,and SⅡ was superior to NLR,PLR and PIV.Compared with NLR≥3.36 group,PLR ≥223.54 group and SⅡ≥769.29 group,the disease control rate(DCR)after immunotherapy was higher in the NLR＜3.36 group,PLR＜223.54 group and SⅡ＜769.29 group(P＜0.01);the progression-free survival(PFS)was longer in the NLR＜3.36 group and SⅡ＜769.29 group(P＜0.05),and the overall survival(OS)was longer in the NLR＜3.36 group,PLR＜223.54 group and SⅡ＜769.29 group(P＜0.05).The univariate analysis showed that Eastern Cooperative Oncology Group Performance Status(ECOG PS)score,NLR and SⅡ were risk factors for the PFS of patients after immunotherapy;the liver metastasis,bone metastasis,NLR,PLR and SⅡ were risk factors for the OS of patients after immunotherapy(P＜0.05).The multivariate Cox proportional risk model analysis showed that SⅡ≥769.29 was an independent risk factor for the prognosis of pMMR mCRC patients after immunotherapy(P＜0.001).Conclusions Peripheral blood NLR,PLR,PIV and SⅡ could predict the efficacy of immunotherapy in pMMR mCRC patients and SⅡ is superior to NLR,PLR and PIV,and SⅡ≥ 769.29 has independent predictive value for poor prognosis in pMMR mCRC patients receiving immunotherapy.