Tuanjie reservoir is a large-scale reservoir in Ningdu county，which located on the upstream of Meijiang river，Meijiang River is the first tributary of Gong River and the secondary tributary of Ganjiang River.Tuanjie reservoir’s drainage area is 412 square kilometers，this reservoir ’s major functions are flood control and irrigation.Analyzing the anti-rainstorm capacity of Tuanjie reservoir is especially necessary for flood control in its downstream area.This paper take Tuanjie reservoir as an example to put forward a new method for analyzing the anti-rainstorm capacity of a reservoir.In this method，Xin’anjiang model is applied for calculating runoff yield and conflux，backstepping reservoir data is used to optimize model parameters.The assumed rainstorm is designed according to the measured data of rainfall.