According to the mechanism of rainstorm flood, we propose a dynamic critical rainfall index associated with the former water levels. The method for calculating critical rainfall of flood hazard is established by using HBV model in the upper Huaihe River Basin. First of all, the optimized model parameters is determined through the model calibration and validation. The stagedischarge relationship is then established for the rising levels of flood. Finally, the critical rainfall is determined according to the hazard water level by repeated model simulation. The case study in the upper Huaihe River Basin indicated that HBV model can well simulate the process of flood response to rainfall after model calibration by using daily meteorological and hydrological data for the period of 2002-2009. The model combined with the stagedischarge relationship is then used to calculate the dynamic critical rainfall over the Wangjiaba subregion. The critical rainfall decreases with the increase in former water level. The variation of critical rainfall with different former water level shows significantly nonlinear response characteristics.