Aiming at the problem that the conventional water level flow reversal method was not considered for the effect of the early rainfall effect，and the accuracy of the small watershed confluence time which determined by the reasoning formula method was not high，the maximum water storage capacity estimation method was used to consider the preliminary rainfall affect.On the basis of this，considering the distributed hydrological model which can reflect the physical characteristics of small watersheds，reliability high convergence time parameters was get.Accordingly，that an improved water level flow reversal method was constructed.The analysis show that the established model and method were superior accuracy which can be used for the calculation of the critical rainfall of the mountain torrent，that can provide reference for the flash flood warning of other small and medium-sized watersheds.