3D object surface reconstruction has important applications in modern clinical medicine, scene modeling and forestry survey and so on. In order to better understand the reconstruction of 3D object surface, this paper first introduces the concept of the Alpha shape of the 3D discrete point set. Based on the analysis of surface reconstruction algorithm using self?adaptive step Alpha?shape is proposed. The value of Alpha is updated dynamically using the kd?tree structure and the average distance of k?nearest neighbors, so that the algorithm can reconstruct the surface with less number of times when the density of the point set is larger. Thus, the reconstruction effect is improved and the operation efficiency of the algorithm is improved. The experimental results with a large number of random data and realistic 3D scanning data show that the proposed algorithm can greatly improve the efficiency compared with the original algorithm.