This study investigated the required strength to fill the cemented backfill by the two-step stage. The mechanical properties of cemented backfill in the goaf were analyzed. It was found that the cemented backfill mainly improved the stress state of the roof rock mass

which provided the support for the broken rock mass of the roof and the lateral pressure for the surrounding rock of the stope

resisted the closure of the stope and limited the flow of tailings. Therefore

a compressive strength model was established under the dangerous mechanical environment for cemented backfill（one side is exposed and the other side bears the pressure of tailings）.This model was based on the collapse caused by shear failure of cemented backfill

and fully considered the factors affecting its stability. In this model

the wedge sliding theory was also used to analyze the limit equilibrium condition of the 3 D wedge-shaped structure with a slip trend on the sliding surface of cemented backfill. The reasonability of the strength model has been verified by the vertical stress measurements and the stability of the 51 exploratory line cemented filling body of 48-54 panel of Dahongshan copper mine. The results provide significant references to the theoretical study of filling mining in-depth and the corresponding application in similar mines.