Abstract: Antecedent effect rainfall is a key factor in debris flow forecasting, but the current method used to calculate this factor has the defect that the parameters determination of the method is too subjectively. In order to solve this problem, a hydrology-process (e.g. rainfall interception, rainfall infiltration and evapotranspiration) based method to calculate the antecedent effect rainfall for debris flow forecasting is proposed. Taking the antecedent effect rainfall calculated by the hydrology-process based method and the empirical based method as the initial value, the debris flow forecasting system for Sichuan province based on the water-soil coupling mechanism is employed to forecast debris flow disasters induced by the heavy rainfall on July 9, 2013 in Sichuan province. The forecasting results show that the hydrology-process based method that used to calculate the antecedent effect rainfall is more suitable for debris flow forecasting, because the failure prediction rate of debris flow forecasting is 21.1% lower than the method of the empirical formula of the rainfall attenuation. Consequently, the hydrology-process based method that used to calculate the antecedent effect rainfall can serve the debris flow mitigation better.