Abstract: Taking the transition from openpit to underground mining of Mengjiabao iron mine as engineering background according to the plastic zone connection， computation divergence and displacement discontinuity as instable criterion of landslide， the strength reduction method and FLAC3D were used to calculate slope safety factor， determine potential slip surface of the slope and analyze slope stability under the condition of static and dynamic underground mining disturbance. Depending on the slope failure law under the different mining depth， the mining scheme was optimized. The results shown that the slope safety factor without the mining disturbance is 194， meeting stability requirement. In process of underground mining using nonfloor pillar sublevel caving stoping， the thickness of covering layer must reach up to 25m. It plays an important role in absorbing stress， transferring stress and slowing the energy dissipation speed.

Key words: transition from openpit to underground mining, slope stability, strength reduction method, safety factor, FLAC3D