It is vital to forecast the areal precipitation of the dammed lakes’ upper basines and the upper reaches of the rivers in the situation with the water level rising, which threats to the safety of life and property after the occurrence of the Wenchuan Earthquake. In the paper, it is realized the boundary extractions of the upper basins of Nanba, Tangjiashan, Xiaojiaqiao and Laoyingyan dammed lakes and the upper reaches of Jialing River, Minjiang River, Fujiang River, Tuojiang River and Qingyi River by using the DEM model, river map and the locations of dammed lakes based on the GIS system. The areal precipitation is calculated with the forecasting precipitation data of the weather stations in disastrous areas by the Thiessen polygon method. It is established a method of areal precipitation forecasting technology under the condition of incomplete information of basin characteristics available as well as emergency states, and it provides the technical support for decision making service on hydrometeorology.