Rainfall is an important factor triggering mudslides. Existing models of debris flow warning are more of semi-quantitative methods which divides different alert level to forecast disasters and it cannot calculate the mudflow’s exact volume at any moment. By analyzing the precipitation of five major debris flow happened in Wenjia Gilly, introducing debris flow driving rainfall index RTI and applying Weibull distribution altogether to determine the limit precipitation of Wenjia gully, QingPing town, MianZhu city, to propose a model for calculating the instant probability of debris flow at any precipitation, thus obtaining the Wenjia gully’s relational expression between mudflow outbreak probability and its deposit volumes based on results of Weibull distribution and volumes of mudslide deposit. This model is not only to achieve a dynamic quantitative -warning of Wenjia gully debris flow, but also give the scope of debris deposits at any probability. Eventually this model was verified by the mudflow event happened in Wenjia gully on August 13th 2012. It can be safely concluded that Weibull distribution can be effectively applied in debris flow’s early waring and this method should be expanded to other waring analyses of geo-disasters