[Objective] To establish a flood warning model for the mountainous area of Beijing City based on vertical-mixed runoff model in order to provide theoretical basis for the calculation of critical rainfall.[Methods] Considering the weather conditions in Beijing City, vertical-mixed runoff model was used to simulate runoff generation, unit hydrograph and linear reservoir method were applied to simulate overland runoff concentration, Muskingum method to simulate flood routing, and submerged discharge was used to represent safety discharge. The parameters of a flood warning model in the area can be determined by field experiment and calibrated by historical data.[Results] Coefficient of determination, peak-flood discharge, peak time and aging level conformed to the requirement and regulation of mountain flood forecasting through the validation of historical precipitation and hydrological data. In addition, the predicted possibility of a flood is in accordance with reality.[Conclusion] The accuracy of this model is good and can be applied to the early warning of flood in Beijing mountainous area with confidence, thus reducing casualties and property losses. This model also lays a foundation for the calculation of critical rainfall of flood in the mountainous area of Beijing City.