The invention discloses a GIS-based X wave band areal rainfall radar data extraction method which comprises a step of data storage, a step of converting polar coordinate data into raster data, a step of radar rainfall image display and a step of radar rainfall data extraction. Via the GIS-based X wave band areal rainfall radar data extraction method, radar data of polar coordinate projection is converted into grid space data, radar polar coordinate data can be converted into and displayed as areal rainfall data, rainfall values can extracted based on given points and faces, and a data source can be provided for a next step of accurate hydrological analysis; a foundation is laid for applying radar rainfall to routine hydrologic rainfall monitoring, urban flood disaster mitigation, middle and small river basin mountain torrent and mud-rock flow early warning and water resource management.