Abstract: In this paper, the formuals for claculating the critical depths of kinds of rock masses going intotheir potential plastic state are given out ; the calculating methods of the rock stresses abore andbelow the critical depth are presentedl then a case is given to show the different results betweenthe author's calculating method and the traditional one ; finally, employing those research conclusions,the real reason of having a great pressure in soft rock tunnels lying in a deep place is analyesd,a proposal approving the boundary conditions of stress in analoguing the underground rockengineering is put forward.