Abstract: In order to ensure the smooth transition of mining from caving method to filling method in the same level， a vertical isolation pillar shall be set at the transition section. Taking the trapezoidal isolation pillar of -170 m in Longdong mining area of Daye Iron Mine as the study object， it is simplified as a fixed beam with variable moment of inertia， and the initial width of the upper end of the trapezoidal isolation pillar is preliminarily determined based on the allowable value of horizontal strain of the pillar. Based on the initial width of the upper end of the trapezoidal isolation pillar，numerical simulation method is used to calculate the settlement of roof surrounding rock of the filling stope under the synergistic effect of the different width of upper end of the trapezoidal isolation pillar and the different cement-sand ratio of backfill， and data fitting between the upper end width of the trapezoidal isolation pillar and the maximum settlement of the roof surrounding rock under the different cement-sand ratio is carried out. From the allowable settlement of roof surrounding rock， a reasonable width of the upper end of the trapezoidal isolation pillar was obtained， which was matched with the cement-sand ratio of the backfill. The numerical calculation results show that the reasonable width of the upper end of the trapezoid isolation pillar matching backfill with the cement-sand ratio of 1∶4，1∶6，1∶8，1∶10 and 1∶12 is 16 m，19 m，21 m，25 m and 29 m respectively. The above study results can provide a reference for the determination of reasonable width of the transition isolation pillar in the same horizontal caving method to filling method.

Key words: Isolation pillar, Transition from caving method to filling method, Cement-sand ratio, Settlement of roof surrounding rock