Abstract: The formation mechanism and stability of the “combined cantilever beam with inverted bench” structure in fully-mechanized sublevel caving face of ultra-thick seam are studied by a combination method of theoretical analysis and analytical calculation, then the formation mechanism of the “combined cantilever beam with inverted bench” and its two kinds of instability conditions were proposed. Due to two kinds of instability conditions and its influencing factors, it is concluded that the larger the thickness of a fully mechanized sublevel caving mining is, the more likely it is to be unstable when the cantilever beam with inverted bench is bent down. Finally, the pressure and its strength of the supersedure working face were effectively reduced by decreasing the length of “combined cantilever beam with inverted bench” through deep hole infusion ahead of the working face in the roadway, improving the support strength and stability of supports by optimizing its parameters and properties.