Abstract: As the main factor affecting the ecological stability of urban green space, the urban green space water system plays an important role in stormwater management. Summarizing the design methods with universal guiding significance can provide reference for the theoretical research and construction of sponge cities. Taking the 2nd Hebei Garden Expo (Qinhuangdao) Park as the research object, this paper proposes a design method of water system in urban green space based on the water security objectives by analyzing and solving rainwater safety problems of the site. For the site’s water system design based on water security objectives: 1) Determine the objectives of stormwater runoff control; 2) Calculate the total amount of runoff reduction from sources in and out of the site, and propose the volume of water storage; 3) Establish a dynamic management model with SWMM to compare and analyze the stormwater storage capacity before and after the site development; 4) Simultaneously meet the water safety objectives, combining the landscape strategy of water system, through the water conservancy analysis model (MIKE21) to simulate the waterscape effect. The conclusion comes to: 1) Through reasonable water system design, urban green space can effectively regulate and store surface runoff in the site and surrounding areas; 2) In the design of urban green space water system based on water security objectives, the current conditions, sponge city regional construction requirements and control objectives should be taken into comprehensive consideration, and the final design scheme should be determined by combining quantitative and qualitative analysis with the scenario simulation method.