Objective To investigate the status and influencing factors of ambivalence over emotional expression (AEE) in breast cancer patients based on the ABC-X model, and to provide a reference for developing targeted psychological intervention strategies. Methods An explanatory sequential mixed-methods design was adopted. A convenience sample of 400 breast cancer patients was selected, then they were surveyed using the Ambivalence over Emotional Expression Questionnaire (AEQ), the Chinese Perceived Stress Scale (CPSS), the Social Support Rating Scale (SSRS), and the Irrational Belief Scale (IBS). Multiple linear regression analysis was used to analyze influencing factors. Based on quantitative results, 15 patients with AEQ scores≥23 points were selected for qualitative interviews, and the thematic framework analysis was applied to analyze the interview data. Results The participants′ total score of AEQ was (37.61±18.23) points. Results of multiple linear regression analysis showed that, pain level, perceived stress, social support, and irrational beliefs were the main influencing factors of AEE (all P<0.05). Qualitative analysis identified four themes:perceived multiple pressures, treatment-related physical and psychological distress, lack of social support and social stigma, and strong irrational cognitions. Integrated mixed-methods results revealed that influencing factors of AEE showed complementarity, consistency, and expansion in stressor factors; complementarity and expansion in resource factors; and complementarity in cognitive factors. Conclusion Breast cancer patients experience a moderate level of AEE, which is influenced by multiple factors. It is recommended that healthcare providers reduce multiple stress experiences, comprehensively manage treatment-related physical and psychological distress, build effective multi-source support systems, identify and correct irrational beliefs,and thereby alleviating patients′ AEE and promoting their physical and psychological recovery.