Objective To explore the mediating role of psychological capital between well-being and symptoms of anxiety and depression among resident physicians with standardized training（ hereinafter referred to as "standardized training resident physicians"）. Methods This study was a cross-sectional study. Using convenience sampling， standardized training resident physicians from 17 national Western medicine training bases in the Inner Mongolia Autonomous Region were selected as study subjects in October 2024. Assessments were conducted using the Psychological Capital Questionnaire-24（ PCQ-24）， 5-item World Health Organization Well-Being Index（ WHO-5）， and Patient Health Questionnaire Anxiety and Depression Scale（ PHQ-ADS）.Spearman rank correlation coefficient was used to analyze the correlation between different variables and further explore the mediating role of psychological capital and its four dimensions in the relationship between wellbeing， and symptoms of anxiety and depression. Data analysis was conducted using SPSS 27.0 and R 4.5.2 software. Results A total of 3 376 questionnaires were distributed， and 3 306 valid questionnaires were returned， resulting in a valid response rate of 97.93%. Among 3 306 participants， 1 332 participants（40.0%） exhibited symptoms of anxiety and depression（ PHQ-GAD score ≥ 10）. Correlation analysis showed that the PHQ-ADS score was negatively correlated with the PCQ-24 and its four dimensions scores， as well as with the WHO-5 score（ r=-0.414，-0.360，-0.384，-0.412，-0.413，-0.699； all P＜0.01）， and the PCQ-24 and its four dimensions score were positively correlated with the WHO-5 score（ r=0.417，0.375，0.395，0.409，0.404； all P＜ 0.01）， and all differences were statistically significant. Mediating effect analysis showed that psychological capital is composed of four dimensions： self-efficacy， hope， resilience， and optimism partially mediated the relationship between well-being and symptoms of anxiety and depression， with statistically significant differences （P＜ 0.001）. The mediating effect test showed that the mediating effect values of psychological capital between anxiety and depression symptoms and well-being was -0.040 ， accounting for 9.5% of the total effect（ P＜0.001）. Conclusions Well-being not only directly influences symptoms of anxiety and depression among standardized training resident physicians， but also indirectly influences these symptoms through psychological capital and its four dimensions. Enhancing the psychological capital of standardized training resident physicians（ particularly hope， resilience， and optimism） can effectively buffer the negative impact of anxiety and depression symptoms on well-being， thereby playing a mediating protective role.