Abstract: Map overlay is an important spatial analysis in geographic information systems（GIS）. First‚it finds out intersection points of line segments‚then uses the matching principle of “import-export intersection point” to form the result arcs and further realizes the dua-l loop algorithm of polygonon-polygon and line-on-polygon overlay analysis.The original quad-tree indexing structure is improved to handle space split and node assignment‚resulting in enhancing the computation speed.This algorithm has been applied to the software development of commercial geographic information systems.