Generalization of the building polygon maps is a challenging problem within the world range. The paper discusses the automatic generalization algorithm of urban building polygon in GIS environment. The main research focuses on the rectangular adjustment of building polygon, partial concave and convex simplification of the large polygon, partial exaggeration of the small polygon and polygon cluster aggregation. In the practical development, we sort the different building polygon by its shape and characteristic, and simplify it with different methods. The experiment proves that the building polygon simplification products a reasonable result through this algorithm and the morph feature of block has been well preserved.