In the premise of supposed disaster standard, the hydrological model is applied to simulating the relations between rainfall and runoff, getting required rainfall for different base water levels that reach disaster standard, and a new method, new ideas for critical area rainfall calculation of small and medium river valleys are discussed. The research takes the Zhanghe Reservoir, Jingmen, Hubei Province as an example, adopts the Xin’anjiang hydrological model, and determines hydrological model parameters through the simulation of historical cases from 1956 to 2012. Based on the simulation result and reservoir flood capacity, the research simulates and calculates the flood critical area rainfall (to reach the critical area rainfall that the flood limit water level needs) under the conditions of different baseline levels, and different rainfall distributions. The results show that it is feasible to use hydrological model to calculate critical area rainfall so that the calculation method of critical area rainfall in small and medium river valleys can be effectively richened.