Abstract: In order to prevent the safety accident caused by the sudden collapse of the residual slope in a transition from open-pit to underground mining， based on the background of the research on the transition from open-pit to underground mining project of Mengjia Iron Mine， the variation law of the stress field， the displacement field and the plastic zone distribution in the surrounding rock of the residual slope of open-pit， and the influence of the overlying rock layer on slope stability， with the increasing of underground mining depth， are studied by using FLAC3D numerical simulation method. The results show that: ①the overlying rock layer can absorb and transfer the stress of the residual slope in opencast mine， which can buffer the energy release and then support the slope and surrounding rock； ②with the increasing of the underground mining depth， keeping the plane of overlying rock layer at a stable elevation can effectively improve the stress field and plastic zone distribution in open-pit slope， and restrain and constrain the deformation of surrounding rock of slope to a certain extent.

Key words: Transition from open-pit to underground mining, Underground mining depth, Residual slope of open-pit, Stability, Numerical evaluation.