Abstract: Based on the geomorphologic unit hydrograph,the Philip infiltration equation and the rainfall property frequency distribution,this paper presents the model for the estimation of the theoretical frequency distribution and return period of the peak discharge less than or equal to the given Q_p through the derivation of the probabilities of rainf all events without yielding runoff using the concept of initial time of runoff and excess rainfall.This paper also gives the examples of the model test and analyzes the prospect of application of the model.