Surface reconstruction by alpha geometric shape based on triangulation is a new idea and method in computer graphics and reverse engineering. In the process of Topology reconstruction, the points are reduced and projected to a middle slice plane. The adjacency among slices is established by section line subdivision, and triangle mesh is stored and called by the form of STL. The simplexes are classified and computed in order to obtain α interval by the external-ball radius and center of every simplex, and the data structure is designed to save them. Through the man-machine interactive selection modes, α-shape can be reconstructed by different α parameters. The surface reconstruction is completed.