Abstract: The research on the accessibility of urban park green space based on the balance of supply and demand plays an essential role in optimizing the layout of urban green space and improving the service level of park green space. Aiming at the problem that the traditional two-step floating catchment area method (2SFCA) fails to fully consider the humanistic perspective at the fine spatial scale, this research improves the 2SFCA model from both supply and demand aspects. It applies the improved model to the accessibility evaluation of urban park green space. Specifically, as to the supply-side improvement, the research adds the diversity index of service functions within the service scope of park green space to measure the attractiveness of parks; as to the demand-side improvement, the research introduces the Huff’s model and considers the probability of people’s subjective choice of park green space. Taking the area within the Third Ring Road in Chengdu (hereinafter referred to as the Area) as an example, the research analyzes the spatial distribution and supply and demand status of park green spaces in the central urban area of Chengdu. The results show that: 1) in terms of spatial distribution, the accessibility is “high in the west and low in the east” overall; 2) areas with high accessibility are clustered and distributed in clumps; 3) the supply and demand relationship in the Area is not ideal, with the total area of areas with insufficient or deficient supply therein accounting for 89.49%; 4) areas with good supply and demand are concentrated in Dongpo Sub-district and its surrounding areas. Based on evaluation results, the research classifies areas with imbalanced supply and demand, and proposes pertinent optimization strategies for short-term micro-renewal, mid- and long-term planning, and long-term adaptation.