Abstract: By introducing numerical manifold method with two sets of analytic grid, i.e., the net of mathematics and physics covers and making use of its maneuverability for disposing of the question of simulation of excavation, this paper emphatically solved numerical manifold method in simulation of excavation. Then, the evaluating method of the stability of karst roof in roadbed had been established by combining with strength reduction technique and numerical manifold method. On scuh a basis, the determination method for safe thickness of karst roof in roadbed was proposed by studying the relationship between karst roof stability and depth or geometry shape. The method was applied to a practical engineering project and had been proved successful.