Background: In response to the United Nations Sustainable Development Goal (SDG) 3.4 (mental health target), this study aimed to explore the facilitating and barrier factors influencing doctors’ willingness to participate in the Mental Health Gap Action Programme (mhGAP) training in tertiary hospitals, using the Consolidated Framework for Implementation Research (CFIR). Methods: Semi-structured interviews were conducted with 20 physicians from tertiary hospitals. Content analysis was performed using NVivo 11, and results were reported in accordance with the Consolidated Criteria for Reporting Qualitative Research (COREQ). Results: A total of 23 facilitating factors and 13 barrier factors were identified, distributed across five CFIR dimensions: innovation, outer setting, inner setting, individuals, and implementation process. Results showed that the implementation process dimension contained the most facilitating factors (n = 7), while the outer setting dimension had the most barrier factors (n = 4). Conclusions: The study indicates that mhGAP training has strong promotion potential in tertiary hospitals. However, targeted optimization of external support systems and implementation details is required to enable mhGAP training in tertiary medical institutions to play a leading, supportive, and radiating role in alleviating mental health resource scarcity and optimizing mental health services.