Abstract: After closing the deep sunken open pit of Lunan Mining Company，the technique of tailings backfilling into open pit is proposed to face with the insufficient storage capacity of the tailings pond.ABAQUS definite element numerical software is used to simulate the end of the open pit mining，dry tailing dumping at bottom，and the first phase mining of transiting from open pit to underground.The changing trend of open pit slope displacement，as well as the stress and strain distribution of mining area during transition from open-pit to underground is analyzed.It is concluded that the stress and strain of underground mining area has no obvious increase after the tailings backfilling into the bottom of open pit.Meanwhile，plastic zone of surrounding rock is not well run through.The horizontal and vertical displacements of the open-pit slope are under control.In view of water accumulation at pit during transition from open pit to underground mining，the water seepage and prevention measures are put forward，which have a better engineering reference value.

Key words: Deep sunken open pit, Tailings dumped into open pit, ABAQUS, Stability analysis