Space borne laser altimeter is able to measure the distance between satellite and land surface. Combining with the attitude, orbit and equipment placement information, the elevation of the measured area can be calculated, which can be used as an elevation control point in mapping. By the influence of satellite orbit height and laser spread angle, the footprint of laser on the land surface is in diameter of tens of meters. For the situation of a flat surface, every point has equal elevation and the central point of the footprint can be chosen as an elevation control point. For the situation of undulating surface, each point has different elevation, and there will be much error if the central point of the footprint is chosen as an elevation control point. Now, the point in latter situation is excluded which makes the efficiency of laser altimeter low. In this paper, the situation of undulating surface is considered, and new approaches of using DSM and image recognition are adopted, resulting in acquiring elevation control point in this situation and increasing the efficiency of laser altimeter.