Abstract: A good areal rainfall calculation means we can forecast flood more accurately and in time.Here,we propose an areal rainfall forecasting methodology integrated fuzzy optimized neural network with Geography Information System(GIS)methods.GIS has advantage of process spatial information.Using many models and methods provided by GIS software,we obtain more accurate areal rainfalls of a catchment.Then,these outputs of the GIS software are taken as the expected output of the fuzzy optimized neural network,and the network is trained to find the mapping between the areal rainfalls and observed rainfalls of all gauge stations. Finally,with the mapping,new observed values are taken as input of the network,and we can obtain the catchment areal rainfall in time.