Based on the principle of the rainfall station sampling method, the probability distribution of the true value of areal rainfall was deduced, using the estimated areal rainfall, in order to describe the uncertainty of areal rainfall calculation under the conditions of existing rainfall stations. Then, probabilistic flood forecasting was performed in combination with deterministic prediction models. Application of this method to the Huangnizhuang Basin of the Huaihe River shows that it can obtain estimates of the probability distribution of the true value of areal rainfall for any periods of time in the basin and describe the uncertainty of the areal rainfall calculation. Furthermore, using a hydrologic model(e. g. , the Xinanjiang model)and the Monte-Carlo sampling technique, the probability distribution of the predicted flow rate can be estimated, and probabilistic flood forecasting can be carried out.