In this paper,an analysis method of catchment runoff is put forward to forecast river flood,which only uses the observed data from the hydrological stations of the rivers,with the characteristics of clear and definite concept,rigorous calculating approach and higher forecasting precision.This method is very suitable to the flood forecasting for the larger rivers with many hydrological stations and long series of flow data.It has been used in the flood forecasting for the upper reaches of the Huaihe River and the good results have been achieved.The application of the method will contribute a lot to the flood prevention,water control and water resources utilizing of the larger rivers of China.