Abstract: With the present increasing demand on mineral resources，enhancing mining strength and decreasing shallow resources，open-pit mines are going towards the depth and transiting to the underground mining，with many foreign and domestic mines now being in the open-pit and underground combined mining status.Therefore，how to determine the thickness of the reservation layer in such a mining system is an important yet to be solved problem.Using numeric analysis and according to the displacement convergence condition at the same point above the mining room for the same cross section at different roof  positions，the displacement convergence criterion for the rational thickness of the reservation layer in an open-pit and underground combined mining system was established，which was used in the analysis of a large mine，where the safety isolation layer was determined for the system，providing a technical basis for the safety mining.

Key words: Open-pit and underground combined mining, Rational thickness of reservation layer, Displacement convergence method