Referring to the methods of hydrological sections and meteorological observatories all over the country,the boundaries of seven great river valleys (including Changjiang River,Yellow River,Songhuajiang River,Liaohe River,Haihe River,Huaihe River and Zhujiang River) and their sub-valleys are objectively determined.The whole country is divided into 71 sub-valleys and computer automatically selects the representative stations of each sub valley.Having studied the advantages of all kinds of methods calculating area rainfall,Taisen Polygon Method as the main method is chosen to calculate area rainfall.The tryout of area rainfall forecast was made in the Central Meteorological Obervatory (CMO) during the period from June to September 2000.On the basis of timely incorporating 24 hour conventional rainfall data and intensive rainfall data as the real time data every day and converting 24 hour and 48 hour short term rainfall forecast guiding products offered by the CMO into station rainfall,real time and forecast area rainfall of each sub-valley are calculated.And at the same time,displaying real time and forecast area rainfall under the MICAPS system is realized.