Abstract: After the open-pit mining of the Mengjiapuzi Iron Mine is completed，the ore body below the level of +70 m is mined by the sublevel caving method.As the underground mining progresses，the overburden has already caved to the bottom of the open pit，formed a large-scale surface collapse pit，the rock movement posed a serious threat to the surface office and shaft.At present，the remaining waste rock cannot meet the filling requirements of surface collapse pits， and the development of rock movement along the side wall of the collapse pit cannot be effectively controlled.In order to solve this technical problem，according to the support principle of critical medium column，this paper studies the new rock movement control method of tailings instead of waste rock filling surface collapse pit，the characteristics of tailings penetration under different tailings concentration and particle size distribution are obtained，the prediction expression of tailings mixed height affected by the drop height of the loose body is given.The study results show that the tailings height of the concentration range of 70%~80% is 130.76~42.36 m，and the mixed height of tailings in this concentration range is smaller than the minimum thickness of the bulk layer in the collapse pit，it is safe and feasible to fill the surface collapse pit with this concentration range. Finally we determine that the reasonable tailings filling concentration on the site is 75%~80%.It is safe and feasible to use the tailings to fill the surface collapse pit， and the reasonable tailings filling concentration is 75%~80%.In order to ensure the safe and smooth operation of the filling operation，the study divides the collapse pit into a slope filling area and a full filling area，and proposes a three-way coordinated filling method along the axial and radial directions of the collapse pit，which provides a strong guarantee for the safe， green and efficient mining.

Key words: Underground mining, Critical medium column, Tailings permeability characteristics, Rock movement control, Surface tailings filling, Synergetic filling