Objectives In Korea, health insurance payment systems are applied differently for mentally ill patients depending on the type of medical insurance. Moreover, medical use differs according to the type of medical insurance. In 2017, the payment system for outpatients with mental disorders under the medical aid program was changed to fee-for-service. This study examined the medical use of outpatients with mental disorders after the policy change. Methods This study used the data of claims from the Health Insurance Review and Assessment Service from 2015 to 2019. A controlled interrupted time series analysis of medical aid and health insurance patients was used as target and control groups. The dependent variables were the number of visits and prescription days for monthly outpatients, which can represent the medical use. Results 5020773 health insurance beneficiaries, and 433435 were medical beneficiaries. The Charlson Comorbidity Index was higher for medical aid patients than for health insurance patients. Before the policy, compared to health insurance patients, medical aid patients had a lower number of hospital visits and a higher number of days prescribed. After the policy, the number of hospital visits for medical aid patients increased (p<0.0001) compared to health insurance ones, and the trend increased (p=0.0070). The number of days prescribed decreased (p<0.0001) and the trend showed a decrease (p<0.0001). Conclusion After changes in the medical payment system, the differences in psychiatric outpatient treatment utilization decreased depending on the type of health insurance.