Abstract: An improved Boolean operation algorithm for surface mesh models is proposed which successfully simulates the dynamic drilling process on bones in virtual ACL （anterior cruciate ligament） surgery．The algorithm adjusts all the triangles on the mesh model that intersect with the cutter surface by shifting their outside vertices onto the cutter surface．These triangles are further subdivided if necessary and the positions of the concerned vertices are adjusted so that the line segments of the shifted vertices can approximate the intersection line under a given tolerance．The surface mesh of the resulting tunnel is dynamically generated with Advancing Front Method．The presented method resolves the T-junction problem in other approximate Boolean operation approaches, and it is able to preserve both the surface smoothness and the sharp features．Experiments show the real time performance of our approach．