Objective To investigate the effects of metformin on cognitive function in schizophrenic patients with diabetes. Methods A total of 100 schizophrenic patients with diabetes who were hospitalized in Jinshan Mental Health Center of Shanghai from January 2016 to December 2017 were selected as the study subjects. The patients were divided into control group and observation group with 50 cases in each group by random number table method. The control group was given atypical antipsychotic drugs as well as conventional hypoglycemic drugs and placebo， while the observation group was given atypical antipsychotic drug as well as conventional hypoglycemic drugs and metformin. The fasting blood glucose（ FPG）， postprandial 2 h sugar （2 hPG）， glycosylated hemoglobin（ HbA1c）， and repetitive complete neuropsychological status test（ RBANS） scores were observed before and after treatment in the two groups， and the adverse reactions in the two groups during the treatment were statistically analyzed. Results The levels of FPG， 2 hPG， and HbA1c in the two groups were significantly lower than those before the treatment（ P＜0.05）. The decrease in the observation group was more obvious than that in the control group at the same time（ P＜0.05）. The scores of immediate memory， visual span， speech function， attention function and RBANS in the two groups were significantly higher than those before treatment（ P＜0.05）， and the increase in the observation group was more obvious than that in the control group at the same time. There were no serious adverse reactions such as hypoglycemia， abnormal liver and kidney functions， the safety of the two groups was good. Conclusions Patients with schizophrenia and diabetes mellitus treated with metformin on the basis of routine treatment can improve cognitive function， with good safety.