Nursing interns play a dual role in clinical practice, acting as both potential risks to and defenders of patient safety. Although moral sensitivity is recognized as a foundational element for patient safety, the specific mechanism translating it into actual safety competency, particularly the mediating role of ethical action, remains underexplored. To examine the mediating effect of ethical action on the relationship between moral sensitivity and patient safety competency among nursing interns. A cross-sectional study was reported following the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines. This study was conducted with 311 nursing interns recruited via cluster sampling from a tertiary Grade A hospital in central China (Henan Province). Data were collected using the Moral Sensitivity Questionnaire, the Ethical Action Subscale of the Judgment About Nursing Decision, and the Patient Safety Competency Scale. Structural Equation Modeling was employed to test the hypothesized relationships and the mediating effect. Moral sensitivity was positively correlated with both ethical action (r = 0.509, p < 0.001) and patient safety competency (r = 0.480, p < 0.001), while ethical action was also positively correlated with patient safety competency (r = 0.603, p < 0.001). Structural equation modeling demonstrated that moral sensitivity exerted both a direct positive effect (β = 0.34, p = 0.001) and an indirect positive effect through ethical action (β = 0.28, p = 0.001) on patient safety competency, with the mediating effect accounting for 44.8% of the total effect. Ethical action serves as the pivotal mechanism translating moral sensitivity into patient safety competency. Nursing education should integrate ethics curriculum and clinical practice through action-oriented training in authentic clinical settings, thereby empowering interns to convert moral sensitivity into patient safety competency.