Abstract: This paper aims to analyze the slope stability of a deep concave open-pit mine, determine the boundary pillar thickness of the deep concave open-pit mine transiting to underground, and ensure underground mining safety. Based on the engineering background of a metal mine in the final stage of open-pit mining, firstly, the physical and mechanical parameters of the rock mass after engineering weakening treatment are determined through indoor rock mechanics tests and GSI rock quality evaluation indicators. Secondly, according to the principle of “sections with similar engineering geological conditions, geometric shapes of designed slopes, and slope inclinations are designated as the same zone”, the open-pit stope is divided into three engineering geological zones. GEO-Studio and FLAC^3D are used to establish two-dimensional and three-dimensional models of the mining area, and the stability of the current open-pit slope and the slope after the transition to underground mining is studied from the perspectives of limit equilibrium and numerical simulation. Finally, three-dimensional models of the boundary pillars are established to calculate the slope stability under three different working conditions, and the thickness of the boundary pillars is optimized. Some important results are obtained. The limit equilibrium analysis method using simplified Morgenstern Price（M-P） and Bishop methods shows that the safety factors（SF） of all existing open-pit profiles under three working conditions of self-gravity, blasting vibration force, and seismic force are all greater than the standard requirements, which has a minimum SF of 1.477. The SF of the mining site with three-dimensional analysis is 1.74, which is greater than the allowable safety factor of 1.20, indicating the stability of the current slope. The X-direction displacement shows as zone A＞zone B＞zone C, with a maximum value of 30.1 cm. The Y-direction displacement shows as zone C＞zone B＞zone A, with a maximum value of 20.7 cm. Attention should be paid to monitoring the slopes of the zone A and zone C. Through simulation calculations of model excavation and backfilling, the recommended value of the bottom pillar for the reserved boundary for open-pit transited to underground mining is 25-30 m under earthquake and rainfall conditions. The findings provide a scientific basis for the stability analysis of deep open-pit slopes and the determination of boundary thickness for transition from open-pit to underground mining, which has significant engineering practical significance.