Abstract: In the process of transition from open pit to underground mining，slope instability will affect the safety in production seriously.The main reasons are that underground mining will cause stress redistribution，the displacement of rock mass，reduce the strength of rock mass and influence the slope stability.In order to study the impact of the transition from open pit to underground mining on the slope stability，and taking the actual mining project as a case，the numerical simulation systems MIDAS,FLAC3D are adopted to simulate the influence mechanism of slope stability under three different mining locations of slope toe，slope back and out of slope.As the result，underground mining in slope toe will lead to maximum sliding area of cover rocks and large slope instability probability；At outside of slope，the settlement of the upper slope reduce the slope angle，which is good for the stability.At slope back，its slope stability falls in between above.The evaluation on the practical engineering showed that the safety factors of mining at slope toe and slope back are insufficient，but mining at outside of slope is in stable.

Key words: Transition from open-pit to underground, Underground mining, Numerical simulation, Stability