Objective To investigate influencing factors of deficient mismatch repair(MMR)in colorectal cancer(CRC).Methods A total of 808 patients with CRC treated in the First People's Hospital of Huzhou from October 2019 to March 2023 and Huzhou Central Hospital from July 2020 to September 2022 were selected as the research objects.According to the diagnosis age of CRC patients,under 50 years old patients as early onset CRC group(57 patients),and over 50 years old patients as late onset CRC group(751 patients).Expression of MMR protein in colorectal tissue was detected through immunohistochemical staining.Influencing factor of MMR protein expression deficiency was analyzed.Results Rate of MMR protein deficiency was 6.8%.Univariate analysis showed that compared with proficient MMR patients,deficient MMR patients had significantly lower age,significantly higher proportion of lesions in the right colon,significantly higher Ki67 positive cell rate,significantly longer tumor length,significantly worse differentiation,significantly fewer lymph node metastases,and significantly earlier TNM stage(P<0.05).Logistic regression multivariate analysis showed that age,tumor location,tumor length,and Ki67 positive cell rate were independent influencing factors for the loss of MMR protein expression in colorectal cancer patients(P<0.05).Proportion of MMR defects in early onset CRC group was significantly higher than that in late onset CRC group(P<0.05).Conclusion CRC patients with MMR protein expression deficiency are characterized by younger onset age,more right colon,longer tumor length and higher Ki67 positive cell rate.