The runoff coefficient is an important parameter in engineering design.Based on the hydrological data of the 2010 edition of the“Handbook of Rainstorm and Flood Calculation Manual in Jiangxi province”，this paper analyzes the relationship between the designed rainstorm amount and the runoff coefficient of natural watershed in Jiangxi province.When the amount of rainstorm over 110mm，the amount of rainstorm is inversely proportional to the runoff coefficient.By the historical data of the maximum 24-hour rainstorm measured by all weather stations in the province，the runoff coefficient of natural watershed in the province can be obtained between 0.72 and 0.97.At the same time according to their relationship，listed the actual application in the project instance，for engineering design reference.