A great deal of data obtained from production practice indicates that in medium-hard seams and softer seams the resistance of rear leg is smaller than that of front leg in a sublevel caving powered support. When caving the top-coal, the resistances of both front leg and rear leg decrease, but the resistance of front leg decreases more than that of rear leg. In some cases, the resistance of rear leg even rapidly lowers to zero. This will seriously bring bad influence to making full use of the load-carrying capacity of the support, and damage the relation between support and surrounding rocks. So that to damage the control of the surrounding rocks of the face. The theory analysis points out that because the top-coal closed the gob is very fragmentized, lowering of the stiffness brings lowering of the load-carrying capacity, so the support’s acting point of the load moves frontward and thus the resistance of the rear leg is smaller than that of the front leg. Because of the top-coal’s further fracturing and moving the fact becomes more serious when top-coal is caving. The extreme case is that the resistance of the rear leg decreases to zero. Based on the theory analysis, the paper presented some measures to improve the load-bearing condition of the support. For example, changing the structure of the powered support and not caving too much top-coal, etc.