Abstract: Boundary pillar is an guarantee for ensuring the safety of underground mining and open pit and the stability of open pit slope，and the reasonable thickness of boundary pillar is essential to realize the transition from open pit to underground.To determine the safety thickness of boundary pillar in Xinqiao mine，such measure as ratio of span and thickness equation，load transfer method and structural mechanics method，are adopted to calculate and analyze the safety thickness of boundary pillar.Numerical simulation on different spans and different boundary pillar thickness is carried out by ANSYS.By analyzing the theoretical calculations and numerical simulation results，the safety depth of boundary pillar under different goaf span is obtained.The results can effectively determine the thickness of boundary pillar of mine design，and have important implications for guiding safety production in transition from open pit to underground mining.Based on the actual mining span (30 m) in the mine，the safety thickness of boundary pillar in Xinqiao mine is recommended as 24 m.

Key words: Transition from open pit to underground mining, Boundary pillar, Theoretical calculation, Numerical simulation