To achieve the accurate and efficient NC milling simulation based on the discrete triangular mesh model, we proposed an algorithm for geometric modeling and intersection. We construct the R*-tree index for upper-surface nodes of mesh model, based on which the nodes within cutting region can be obtained. We compute tool path segments within cutting projection region of node, and calculate the minimum adjustment height of node according to tool path segments within cutting projection region and then change the z-value of node. Thus, we complete the intersection calculation in simulation process. It has been proved by examples that the algorithm for geometric modeling and intersection in NC milling simulation has strong adaptation to tool path segment type and that it can accurately and efficiently reflect the effect of NC simulation process based on the discrete triangular mesh model of rough.