Abstract: A gold mine,from below 1 385 m elevation,will transition from open-pit to underground mining,intending to use the upward approach and filling mining method.In order to avoid mutual interference caused by simulta-neous open-pit and underground mining and achieve a smooth transition of mine production,it is necessary to study the reasonable thickness of boundary roof pillars.Based on relevant laboratory experiments,the reasonable range of boundary roof pillar thickness was calculated using the K.B.Lu Peinie equation,three-zone theory,and engineering analogy method.Flac3D software was used to numerically simulate different thicknesses of boundary roof pillars to determine the reasonable thickness for the transition from open-pit to underground mining in the mine.The results show that when underground mining is conducted in primary ore and oxide ore areas,boundary roof pillars of 20 m and 25 m respectively need to be reserved,at which point the impact of underground mining on open-pit slopes is minimized and the overall mining area tends to be stable,meeting the requirements for safe mine production.The research results can provide a theoretical basis for mine design.