BACKGROUND: The prevalence of burnout, depression, and anxiety among Chinese nurses was 34%, 55.5%, and 41.8% respectively. Mental health problems have significant impacts on their personal well-being, work performance, patient care quality, and the overall healthcare system. Mental health is influenced by factors at multiple levels and their interactions. METHODS: This was a descriptive qualitative study using phenomenological approach. We recruited a total of 48 nurses from a tertiary hospital in Changsha, Hunan Province, China. Data were collected through focus group interviews. Audio-recorded data were transcribed and inductively analysed. RESULTS: Four major themes with 13 subthemes were identified according to the social ecological model: (1) individual-level factors, including personality traits, sleep quality, workplace adaptability, and years of work experience; (2) interpersonal-level factors, encompassing interpersonal support and role conflict; (3) organization-level factors, such as organizational climate, organizational support, career plateau, and job control; and (4) social-level factors, which included compensation packages, social status, and legislative provision and policy. CONCLUSIONS: Our study provides a nuanced understanding of the multifaceted factors influencing nurses' mental health. Recognizing the interconnectedness of individual, interpersonal, organizational, and social elements is essential for developing targeted interventions and comprehensive strategies to promote and safeguard the mental well-being of nurses in clinical settings. TRIAL AND PROTOCOL REGISTRATION: The larger study was registered with Chinese Clinical Trial Registry: ChiCTR2300072142 (05/06/2023) https://www.chictr.org.cn/showproj.html?proj=192676 . REPORTING METHOD: This study is reported according to the Consolidated Criteria for Reporting Qualitative Research (COREQ).