For many years, the impaction of watershed morphologic and topographic structure and ral variation of rainfall on unit hydrograph has been the key question in studying method of routing. This paper introduced the traditional classifications of unit hydrograph according to the spatio-tempowatershed flow spatio-temporal variation and analyzed their disadvantages. A new S-curve method considering the spatio-temporal variation of rainfall was proposed. The relationship between the parameters of the new S-curve equation and the barycenter of storm and the intensity of rainfall was found. The unit hydrograph with changing position of storm barycenter and intensity of rainfall can be built by using this method, which has the characteristics of Sherman unit hydrograph and also consider the temporal-spatial variation.