Objective To explore the mediating effect of clinical internship adaptation on the relationship between role stress and mental health among undergraduate nursing interns. Methods A convenience sampling method was used to select 450 undergraduate nursing interns from medical colleges in Tianjin City as the participants. They were surveyed using the Nursing Students Clinical Practice Maladjustment Scale，the Role Stress Scale，and the Kessler Psycholigical Distress Scale. Results The clinical internship adaptation score of the 450 undergraduate nursing interns was 43.00（41.00，46.00） points，the role stress score was 103.00（101.00，105.00） points，and the mental health score was 45.00（43.00，46.00） points. The role stress scores of undergraduate nursing interns were negatively correlated with their clinical internship adaptation scores（r=-0.425，P<0.001） and positively correlated with their mental health scores（r=0.528，P<0.001）. Additionally，their clinical internship adaptation scores were negatively correlated with mental health scores（r=-0.449，P<0.001）.The partial least squares structural equation model（PLS-SEM） showed that clinical internship adaptation played a partial mediating role between role stress and mental health among undergraduate nursing interns，accounting for 79.18% of the total effect. Conclusion Role stress not only directly affects the mental health level of undergraduate nursing interns but also influences their mental health by affecting their clinical internship adaptation. Hospitals and colleges should pay attention to the role stress of undergraduate nursing interns，effectively guide them to actively change their cognitive structures，enhance their coping abilities and social support levels，thereby improving their mental health.