The precipitation in forecast period greatly influences flood forecasting precision. The longer the forecast period is, the bigger the influence of the precipitation in forecast period on flood forecast will be, therefore the people more and more pay attention to the precipitation in forecast period. The paper uses the rainfall information that the mesoscale numer〖HJ〗ical model (AREM Model) forecast takes as the precipitation in forecast period, and inputs into the Xinanjiang hydrological model, to make the forecast experiments on flood process in flood season in 2008 in research area. The results show that the flood forecasts can be improved obviously when considering the precipitation in forecast period, and the numerical model forecast will have an expansive application prospect in the hydrology forecast.