This paper introduces how to use GPS on-the-spot survey to update outmoded GIS data in quantitative monitoring of soil erosion.This method involves field survey instruments composed of GPS software and receiving equipment,outline for field survey,conversion of projection of measured data,and updating of outmoded GIS data.In comparison with the RTK technology commonly used in the mapping circle to update land use maps,it is more economical,quicker and more practical,and moreover,its position finding and updated GIS' s data are more reliable after systematic errors are eliminated.The result in the experimental area indicated that the precision of monitoring soil erosion quantificationally was increased 7 percent,comparing the updated land use maps by GPS to the old maps.For increasing the precision of monitoring soil erosion and water quality from nonpoint source pollution,it is attached to update the changed boundary of the land use maps on hill or massif by GPS on-thespot.