Taking an example of Qingyang city, which is a city in the northeast of Gansu Province, weapply Thiessen Polygons method to calculate the surface precipitation by combining the weights of therainfall site, then use the AutoCAD 2008 software to make artificial meshes and apply the ArcGIS 9.3 software to make automatic meshes.At the same time, the calculation result of the surface precipita-tion could be compared with the traditional method that calculate the average rainfall value of all rain-fall site.Based on the results of surface precipitation, we make some relevant analysis on this paper, such as considering the results influenced by the numbers of the rainfall site, the human being's sub-jectivity factors on the aspect of the grid meshing, and the precision of the method.The results of thestudy show that, the surface precipitation calculation result, which based on the Thiessen Polygonmethod, is more close to the real rainfall, what's more, both the numbers of rainfall sites and gridmeshing are the important factor that affects the calculation results.