The information quantity method is used to evaluate the susceptibility of geological hazards, and the whole province is divided into 3 levels. Based on the geological disasters and precipitation data from 2009 to 2014 in Guizhou Province, the relationship between the probability of geological hazards and the effective precipitation at various levels is analyzed. On the basis of this, the dynamic critical rainfall is introduced into the forecast model by the method of probability theory, and the probabilistic forecasting model of geological hazards based on the dynamic critical rainfall is established. The results show that: The critical rainfall of geological hazards is not static, but distributed in a very large range. When the effective precipitation is a specific value in the interval, it is more appropriate to describe the occurrence probability of geological disasters with the corresponding probability. The model prediction accuracy rate is more than 80% in 2015, which shows that the proposed method has practical application value, and can play a good role in forecasting and early warning.