The purpose of forensic psychiatric system is to prevent recidivism of offenders with mental illness by providing psychiatric treatment at a forensic psychiatric center instead of a prison, and facilitate their successful rehabilitation in the community. Despite administering a forensic psychiatric system, the number of offenders with mental illness has continuously increased since 2016, and their recidivism rate was 66.7% in 2020, which implies a serious social problem. However, little is known about the effects of therapeutic interventions at the forensic psychiatric center. Therefore, the present study explored the ways of improving therapeutic effects on interventions for offenders with mental illness by conducting a focus group interview with clinical professionals. As a result of analysis, the present study identified three sub-themes, including ‘psychiatric intervention’, ‘professionality of therapeutic intervention’, ‘professionality of practitioners’ as maintenance factors for improving therapeutic effects. Also, as complementary factors for improving therapeutic effects, this study identified four sub-themes, including ‘recruitment of staff’, ‘empowering practitioners’, ‘systemization of programs’, ‘reinforcement of follow-up intervention’. Based on those results, the study discussed practical and policy implications for preventing recidivism of offenders with mental illness through effective forensic psychiatry.