This paper presents a fast mesh simplification algorithm that combined the half-edge data structure with modified quadric error metric (QEM). When half-edge structure is used, the adjacency queries between components of the mesh, such as vertices, faces and edges, can be quickly achieved and thus the run time is reduced remarkably. Furthermore, with the modified quadric error metric, the quality of the simplified meshes for a certain kind of 3D medical models whose normal vectors are computed through voxel gradient during reconstruction can be greatly improved. The experimental results illustrate the efficiency of the algorithm.