The HIV/AIDS epidemic in China remains a significant public health challenge, necessitating a response that transcends the capacity of any single entity. The national strategy explicitly advocates for a prevention and control framework characterized by “government leadership, multi-departmental responsibility, and social participation” (State Council of the People’s Republic of China, 13th five-year plan for HIV/AIDS prevention and control, 2017). Within this framework, social organizations are pivotal in bridging resource gaps and mobilizing grassroots forces. This study presents an in-depth case analysis of an HIV/AIDS prevention and control project implemented in L District of Sichuan Province, a region severely affected by the epidemic. Utilizing a qualitative case study methodology, including document review, participatory observation, and questionnaire surveys, the research examines a collaborative model led by the District Center for Disease Control and Prevention (CDC) and involving the District People’s Hospital, township governments, community organizations, and volunteers. The analysis delineates the specific functions of social organizations in coordinating intersectoral resources, enhancing service accessibility for high-risk groups (e.g., the elderly, migrant populations), and advancing grassroots practices like door-to-door outreach and interventions in high-risk venues. The study identifies critical deficiencies in the multi-stakeholder collaboration mechanism, such as varying levels of participation willingness among departments, ambiguous responsibility boundaries, and the absence of unified evaluation and supervision standards. Based on these findings, the chapter proposes optimization strategies, including establishing regular communication platforms, clarifying institutional roles, and strengthening participatory and supervisory mechanisms. Furthermore, the study reflects on the conceptual evolution from a narrow biomedical model to a holistic socio-ecological perspective in public health governance. This case offers practical, evidence-based insights for enhancing the efficiency of county-level HIV/AIDS prevention and control and provides a replicable model for promoting the in-depth participation of social organizations in multi-stakeholder collaborative governance, with implications for health policy and grassroots social organization practice across China and similar contexts.