Abstract: This paper discusses the polygon medial axis definition of GIS and calculation geometry,and their main distinguishing feature, pointes out that the polygon medial axis should be the locu of the equal-distance points to two or more edges of the polygon;analyzes the relevant different algorithm and cases in practice, and proposes the geometry means in theory relatively rigid and widely meeting the requirements in practice-dotted pair rank method and the map algebra polygon medial axis method, utilizes them to offer several polygon medial axis calculation examples.