Study on efficient mining of the steep incline and fractured ore-bodies in Yongshaba mine of Guizhou Kailin Group shows that ore-body is fractured and difficult to support the roadways in-vein. After research of the actual conditions about the ore-bodies, we have made the initial decision to adopt reconstruction of roof downward sublevel cut-andfill mining. The men work safely under the false roof supporting the top plate. However, the difficult problem is how to determine the strength of the false roof. In this case, the method based on reliability theory has been put forward. Combined with elastic mechanics and field practice, when practical value of reliable probability is 90 % , the value of the false roof strength has been calculated, and the study shows that stope span greatly influences the false roof strength. With the strength of artificial roof, the reasonable reinforcement design ensures the false roof which can supply the demand of strength under large span and load.