In order to solve the problem that it is difficult to select the water surface rate of waterlogging and rainstorm duration in urban drainage planning and design, a design method based on the coupling effect of two key parameters of water surface rate of waterlogging and rainstorm duration was proposed. In view of the failure of the conventional average elimination method under some working conditions or the small drainage flow obtained, the improvement was made. This method can coordinate the relationship between discharge and storage in the short duration rainstorm and its subsequent precipitation process, quickly determine the design index result table under the coupling condition of water surface rate of waterlogging and rainstorm duration, and then reasonably determine the key parameters and design indexes. Taking Shajing electric drainage area in Nanchang City as an example, MIKE model is used to analyze the difference of drainage process before and after improvement, so as to verify the reliability of the method. The results show that the method can make up for the two defects of the conventional method, realize the rapid calculation of the design index under the coupling condition of key parameters, and solve the problem that the calculation result is too small to adapt to the short-duration concentrated heavy rainfall.