When the same orebody is being mined in a combined open pit and underground mining system, there is an interaction and overlapping of mining inducement which exhibits self-organizing behavior. Using the self-organizing ability of the system and the nonlinear big deformation program of the deviation square FLAC, a simulation was carried out of the combined open pit and underground mining system. Mining-induced slope displacement results were obtained at two different spatial positions. A comparison was made between the maximum slope horizontal displacement and the ultimate displacement at a slope failure so as to determine the reasonable thickness of combined reserve layer, a parameter characteristic of combined open pit and underground mining, thereby ultimately turning the open pit mining into underground mining in a safe and quick way.