Due to the general lack of Hydrological data in small watershed，usually，we indirect Calculation the design flood of small watershed by check the rainstorm parameters according to the Storm flood manual. However，there are many kinds of rainstorm parameters, the traditional manual searching task is arduous and affects the working efficiency. This paper mainly discusses the system design basis, query database establishment method and system function realization method of the automatic calculation of rainstorm parameters system based on ArcGIS Engine.The use of longitude and latitude coordinates is taken into account in the establishment of the database, which improves the accuracy of project positioning. The feasibility, reliability and efficiency of the method are validated by the example of the rainstorm parameter calculation in Jiangxi Province.