Based on Bayesian statistic theory (developed by Roman Krzysztofowicz)which was adopted by the National Weather Service (NWS)of the United States, an academic frame of probability hydrological forecast has been established, which describes the uncertainty of hydrological forecast by using distribution function. According to the frame, the model of probability flood forecast at Xixian Gauging Station of Huaihe River Basin has been studied. The research shows that Bayesian approaches is at least as good as current methods (i.e. deterministic forecast). And it is more valuable than current methods especially when an obvious uncertainty is come forth.