The probability distribution of observed precipitation grades under a certain precipitation grade is analyzed and predicted on the basis of precipitation predictions and observations in Tianjin. From the viewpoint of flood control, the emphasis is put on the possibility of underestimated precipitation events. Adopting the curve fitting method and determining the empirical statistic paramet er from the hydrological frequency distribution curve, the precipitation distributions corresponding to the precipitation prediction of each grade are estimated. Results show that the reliabilities of precipitation predictions are different, and the possibility of underestimated events is bigger than that of overestimated ones for the precipitation predictions of all grades. The procedure to use precipitation predictions for controlling the water level of the reservoir during the flooding season is presented.