[Object] To analyze the relationships among urban green space, catchment area and water point in order to relieve urban storm runoff.[Methods] Based on the latest concept of "sponge city" and ecological stormwater management, we took Zhengzhou City as a case study. The study area was divided into 93 catchment areas. By using water balance method, parameters of green space were calculated.[Results] We summarized the regression relationships between the existing green space and the catchment area, and obtained different green space/water catchment area ratios under different rainfall conditions(50, 100 and 200 mm). The result showed that the regulation and storage effect was the best when the green space ratio was 30%.[Conclusion] Urban green space can be optimized by increasing urban green space scale, reasonable layout and the concrete implementation ways.