Asteroid surface topography surveying and mapping is the first mission of deep space exploration on asteroids. This paper puts forward one method of stereo vision online surveying and mapping the asteroid surface topography when detector approaching observation. It can obtain the 3D terrain of overlapping area by stereo camera,and connect adjacent models by their connection points,determine the shape model and characteristic of the asteroid by means of least squares adjustment. It also puts forward one method of crater feature detection based on contourline analysis,and the crater features in the terrain are identified by extracting and analyzing terrain contour line. Experimental results show that the prototype system can quickly rebuild the 3d terrainof exploring area,identify impact craters from the 3D terrain,and it is proved that the proposed method is practical.