In order to solve the problems caused by the complexity of underlying surface conditions and the uncertainty of antecedent precipitation in mountainous small watershed, two small watersheds, Taibei village in Taiping Town and Dapinggang village in Luojing town of Luoding City, were selected as the research objects. The disaster-causing flow, which is interpolated by the stage-flow relation and the disaster-causing water level, can be used to calculate the critical runoff with the method of triangle confluence curve. With the consideration of the antecedent precipitation, SCS model and Xinanjiang model are applied to calculate the dynamic rainfall threshold reversely, and the dynamic rainfall threshold will be compared with the critical rainfall calculated by the national new technology method used in Guangdong Province. The results show that both models meet the requirements in the calculation of rainfall threshold. Relatively speaking, the results based on the Xinanjiang model are safer and more reliable in the flash flood warning.