Abstract: With the widespread application of bioretention in the construction of sponge city in China, the optimal design of bioretention was paid more and more attention. However, due to the absence of systematically research on hydraulic characteristics analysis and design methods, the design method of urban road grate inlet was often directly adopt in overflow outlet design in the bioretention. However, there was a great difference between the hydraulic characteristics of overflow outlet in bioretention and traditional grate inlet on the road. For the above problems, the hydraulic characteristics of the overflow outlet in the bioretention was analyzed via theoretical methods, and the drainage capacity of which was discussed. The design method of overflow outlet in bioretention was analyzed with an example, so as to provide references for the optimal design of the bioretention overflow outlet.