Abstract: Area rainfall plays an important role in flood prevention and control, storage and discharge of reservoirs, and is an important basis for prediction and evaluation of flood control departments. Taking Luanchuan county of Henan province as an example, it discusses the specific steps of using ArcGIS software to grid the high-precision Dem raster topographic data, establish Thiessen polygon, calculate the area weight of each rainfall station, and then calculate the area rainfall. Through the flood flow simulation results, it can be seen that the Thiessen polygon method based on ArcGIS can improve the work efficiency and the accuracy of calculation results.