The rational distribution of parks within an urban park system should ensure reasonable travel distance for citizens, as well as good recreation quality, which seems to be more important than the former in megacities with high population density. However, studies on the accessibility of parks ignored the competitiveness and exclusiveness of urban green space, and the method can be improved to get a more scientific result as the basis for spatial optimization of urban park systems. Therefore, in this study, we consider the park’s quasi-public goods attribute when building an accessibility measurement method, and both the park’s service supply capacity and demand of citizens were included, as well as the influence of spatial travel cost. This method, based on the empirical research results obtained from a case study of Shaoxing in East China, provides a more suitable accessibility estimate compared with the previous methods, which can reflect the park’s spatial distribution characteristics. Recommendations for improving the accessibility of parks include increasing the number of parks, reducing the cost of travel, enhancing park service capacity, and reducing the population density within the park’s service area.