In order to analyze the impact of the water surface area of a watershed on the design flood,the watershed was classified into a land watershed and a water surface watershed for flood flow calculation at the same time interval.Then,the design flood of the whole watershed was obtained by adding the two flood flows together.Using this method,we calculated design floods with different water surface areas of three reservoirs and analyzed the impact of water surface area on the flood volume and peak flow.The results indicate that larger water surface areas lead to greater impacts on the flood volume and peak flow.For the same watershed area,the impact of water surface area on the flood volume and peak flow is positively proportional to the flood frequency,i.e.,the higher the frequency,the greater the impact becomes. In order to analyze the impact of the water surface area of ​​a watershed on the design flood, the watershed was classified into a land watershed and a water surface watershed for flood flow calculation at the same time interval.Then, the design flood of the whole watershed was obtained by adding the two flood flows together. Using this method, we calculated design floods with different water surface areas of three reservoirs and analyzed the impact of water surface area on the flood volume and peak flow. areas lead to greater impacts on the flood volume and peak flow. For the same watershed area, the impact of water surface area on the flood volume and peak flow is positively proportional to the flood frequency, ie, the higher the frequency, the greater the impact becomes.