Abstract: When underground mines are mined by the open stpoe method or the caving method，the roof will cave.After the caving of the roof strata has developed to the surface，the rock movement will pose a serious threat to the surface industrial facilities and the environment.The theory of critical medium column provides a new idea for surface rock movement control.Taking the mining status of the Xilinhaote Fluorite Deposit with the steeply inclined medium-thick orebody which using short-hole shrinkage method as the engineering background，the main influencing factors of the critical bulk column are studied through field monitoring investigation and similar physical tests.The results show that:① The height of the critical medium column decreases with the increase of the dip angle of the ore body. With the progress of the underground ore drawing， the height of the loose body column is only 39.4%~45.7% of the total height of the bulk，the dense scattered columns above it play a major role in restricting rock movement.② For different ore body thicknesses，by increasing the filling height of the scattered body and reducing the thickness-span ratio，the surface subsidence range can be controlled.③ Proper deformation of the surrounding rock can increase the passive side pressure of the bulk，and reduce the critical medium column height required for the stability of the rock mass of the collapse pit.④ The arch characteristics of the collapse pit are affected by the granularity of the bulk and the collapse pit width，and control the effective height of the critical medium column.The critical arch size range of the Xilinhot Fluorite Deposit is 0.5~0.7 m ，and the critical arch width is 3.5~4.0 m，which is much smaller than the actual distribution width of the collapse pit，that is，the bulk can maintain continuous flow.The above discussion result further indicated that the critical medium column theory can be well applied to Xilinhot Fluorite Mine.

Key words: underground mining, critical medium column, thickness-to-span ratio, rock movement control, granular media arching characteristics