Abstract: Based on the amended and extended GTP model,the underground entity is extracted in levels of geosciences,geometry and topology,and eight geometric primitives including node,edge,arc,triangle,supper-surface,GTP,tetrahedron and super-body are abstracted.The arc refers to an unclosed curve comprised of two or more end-to-end edges,the supper-surface refers to a spatial surface represented by a group of triangles in order,and the supper-body is a spatial region enclosed by a few supper-surfaces or formed by a group of adjacent GTP elements.The geo-entity is 0-3D classified into point entity,line entity,face entity and body entity,and the body entity could be furthermore sorted as simple body entity,compound body entity and complex body entity.A GTP-based entity model(GTP-EM) is presented for underground real 3D integral representation and modeling.With object-oriented method,the topological relations are defined by geometrical elements.With the 3D integral geo-entity representation and modeling for the subway and its surrounding strata located underground,the central business district(CBD) in Beijing city is taken as an example.