Social work service organizations (SWSOs) have developed quickly, and there were nearly 8,000 of such organizations throughout China in the end of 2017. However, this rapid growth has brought both great opportunities and challenges. Globally, leadership has received increasing attention in the social work profession in the 21st century. Among different types of leadership, transformational leadership has been identified as a key issue in bringing more innovation and motivating employees' organizational citizenship behavior to overcome such challenges. Therefore, this study aimed to focus on social workers from the 170 applicants of the “Top 100 Social Work Service Organizations of 2017” which could be seen as the more successful SWSOs in China to examine how transformational leadership impacts organizational citizenship behavior by considering two mechanisms, workplace social capital and professional autonomy, in different areas throughout China. A two‐step process analysis was employed. First, a structural equation model was theoretically built, and it examined the direct and indirect relationships among all four latent variables, considering two mediation effects. Second, multiple group (high social work developed area group versus low social work developed area group) analyses tested the moderation effect. This study confirmed the key role of transformational leadership in the success of SWSOs and found workplace social capital as a full mediator and professional autonomy as a partial mediator in the relationship between transformational leadership and organizational citizenship behavior. These findings may shed light on the further development of SWSOs from the perspective of enhancing social workers' organizational citizenship behavior.