In this paper, the process of failure and falling of the rock masses in the underground mining is studied; and the relationship between the processes of failure and falling of rock masses is proposed based on the rockmass strength theory. Research indicates that the failures of the rock and rock masses are probably not at all in common. Even though the rock masses go into the plastic condition, even to the extent that come into being in the failure, they do not surely fall. Hence, the mechanical properties of rock mass, especially after the failure, still need to be further researched. The laneway and the stope in the mining engineering are the temporary engineering, the load- carrying ability of the rock masses should be bought into play as far as possible, and therefore the stabilization during failure process is a significant for the supporting in the underground mine under safety prerequisite. Since 1 million m3 of the mined-out area is formed by mine for many year at Xishimen iron mine in China, and the gob roof is only 54m from the ground, the mined-out area is potentially dangerous to the underground mine. This paper analyzes the pattern of the stability and falling process in the mined-out area based on the theoretical analysis and numerical calculation, in order to obtains the falling criterion of the roof rock masses and stabilize the mined-out area. The predicted falling shape and range of the mined-out area is compared well with the in-situ observations. This contribution has also been popularized in many mines of China.