Abstract: For zooming in or out the map in a real time GIS application,and for quick transportation of the vector map in a network,this paper proposes the on-line map generalization concept.It discusses the on-line simplification to the line object in detail.Two algorithms are proposed.Their Complex are all from θ (1) to θ(n).It is a great progress comparing the normal DP algorithm whose complex is from θ(n) to θ(n²).The discussion about the reducing the spatial complex is also involved in the paper.