Background: Senior nursing interns frequently encounter ethical challenges due to the complex healthcare environment. In order to effectively address these challenges, ethical decision-making ability is particularly important. Identifying their ethical decision-making ability level and influencing factors is crucial for designing targeted interventions. Objective: To determine the level of ethical decision-making ability and identify its influencing factors among senior nursing interns. Design: A cross-sectional study. Methods: A total of 1096 senior nursing interns from 44 universities across seven regions in China participated in the study. Data were collected using a general information questionnaire, the Judgement About Nursing Decision scale, the Chinese Moral Sensitivity Questionnaire-Revised Version, and the Hospital Ethical Climate Survey. Descriptive statistics, t-tests, one-way analysis of variance, Pearson's correlation, and multivariate linear regression were conducted using SPSS 27.0 for data analysis. Results: Of the 1096 questionnaires, 935 were valid (85.3% response rate). The participants' mean scores for ethical decision-making ability, moral sensitivity, and hospital ethical climate were 276.41 (SD = 18.51), 42.01 (SD = 7.61), and 102.17 (SD = 14.63), respectively, reflecting moderate levels. Pearson's correlation revealed a positive relationship between the number of ways to acquire ethical knowledge and ethical decision-making ability (r = 0.077, p < 0.05). Regression analysis indicated that significant variables collectively accounted for 15.8% of the overall explanatory power on ethical decision-making ability (R-2 = 0.158, P < 0.05). Conclusion: Senior nursing interns exhibited moderate ethical decision-making ability. To enhance this ability, educators are encouraged to develop targeted interventions, such as increasing opportunities for senior nursing interns to acquire ethical knowledge and enhancing their moral sensitivity via educational programs. Clinical institutions should foster a positive hospital ethical climate. These measures are crucial for improving the quality of care.