Abstract: Due to open pit pressure limit，second phase project at underground can not keep stable till the end of stope mining in Dahongshan iron mine，and the mining method can not be transferred smoothly because of the ground pressure.In order to study on reasonable pressure ore area at open pit，the similar material simulation experiment is made to simulate the pillarless sublevel caving method in Dahongshan Iron Mine.According to the simulation of deformation，stress and displacement data in the process of the strata movement，combining comprehensive analysis on on-site data，the caving forms at gap，failure form at open pit and 70° pressure ore angle of open pit were gained.The reasonable pressure boundaries at open pit were redrawn by the 70° pressure ore angle，and the second phase project can last another two years in a safe condition till the open pit mining and its limit is over and smooth transition from pen pit and underground mining will be done at the same time.The study provides a good theoretical guidance to ensure mine safety with reasonable coordinate mining，and provides reference for the joint mining of corresponding surface and underground mines.

Key words: Strata movement, Similar simulation, Pressure ore angle, Surface and underground combined mining