Abstract: Based on the explanatory framework of “motivation - behavior - mechanism”, through case studies and participant observation methods, it is discovered that the expansion of county-level sports administrative departments in China mainly originates from multiple motivations at the management end, the authority end, the demand side, the supply side, and the operation end. The reform of county-level sports administrative departments adopts two forms: the implicit mode (personnel secondment and entrusted management) and the explicit mode (sports commissioners, social sports inspectors), presenting characteristics such as high dependence on talents, strong demand for systems, and full flexibility in approaches. The expansion mechanism of county-level sports administrative departments encompasses the orientation mechanism of taking root at the grassroots level to enrich grassroots sports governance forces; the working mechanism of driving the functions of infrastructure with services; the incentive mechanism for internalizing external professionals; and the digital empowerment mechanism for governance upgrading. The expansion of county-level sports administrative departments should adopt reform models that are adapted to local conditions under the dual constraints of systems and resources to enhance the capacity of grassroots sports governance.