This paper proposes a new strategy of extracting boundary points from scanning point cloud (SPC) data of sheet metal parts (SMPs). This strategy is suitable for bending SMPs with slowly changing surfaces. To cope with the problem that the SMP is too thin to have enough points of its lateral surface to be calculated for the boundary outline, the boundary points are obtained by moving ridge points which is the maximum curvature points on the marginal of parts along theoretical position direction. In this article, the strategy is explained firstly and then carried out on two different experimental SMPs. The strategy contains several steps. Firstly, we construct a slice set called multiple direction slices (MDS) along a curve fitted by boundary points of SPC. Then marginal point data (MPD) is obtained completely and accurately by MDS. And then the chamfer arc data is extracted from MPD by setting identification model of chamfer arc’s two endpoints. Then the ridge points which are the maximal curvature points of chamfer arc data are picked out from chamfer arc data. Finally, by moving the ridge points along a certain direction for a fixed distance, the boundary points are calculated out. Two experiments are carried out to identify position error and form error of the extracted boundary points. The measurement results of boundary outlines of a 6mm thick SMP from a three coordinate measuring machine (CMM) is taken as reference in the first experiment. The second experiment regards theoretical boundary outline as reference. Both two experiments demonstrate the effectiveness of the strategy.