Area feature boundary is one of cartographic objects, its simplification is very important in map generalization. There are many existent methods of boundary simplification. But, in maps at large scale, boundary as right angle is a special cartographic object. There are not any methods used in this kind of cartographic object generalization. A special method must be given for this cartographic object. In this paper, some rules of simplification of area feature boundary and the method of getting spatial knowledge are discussed, such as maintaining area and shape of area feature. The area of boundary can be meintained with respect to calculation of area of added or deleted small parts. The special poins, such as points, such as points on convex hull, are used to control the shape of boundary. The spatial knowledge of boundary can be acquired with respect to computational geometry, and can be represented with the rules and the set of characteristic points. This spatial knowledge can control the map generalization operators. This paper focuses on the progressive method of graphic simplification of area feature boundary as right angle based on its characteristics. The smallest unit used for area feature boundary generalization is rectangle. The rules of establishing rectangle are shown in this paper. In every step, the smallest rectangle is deleted according to the rules. For special case, the boundary data must be handled specially. This idea can be used to other cartographic features. Progressive graphic simplification method can control all details of cartographic object to be generalized. So the sudden change in map generalization can be got from the progressive change, using this method.