Abstract: Stress increment induced by moistening, softening and swelling effect of swelling soils in tunnel are not considered in traditional calculation theory for loosening pressure of surrounding rock. Based on the Nanjing Pingshan tunnel in soft swelling soil, improvement was conducted for the traditional calculation method and a“limit equilibrium numerical method”was proposed considering the impact of softening of rock mass strength and swelling due to moisture increasing in light of Terzaghi theory. The finite difference software FLAC3D was used to analyze the variations of surrounding rock stresses before and after swelling and softening due to moisture increasing, and the effects of tunnel buried depth, rainfall influential depth and swelling rate of surrounding rock were also studied. The results show that loosening pressure of rock mass can be calculated by the“limit equilibrium numerical method” and significantly affected by softening and swelling of rock mass; loosening pressure of surrounding rock increases with an increase of affected depth of rainfall and swelling rate of rock mass; correction factor of the calculation formula for loosening pressure of rock mass is directly proportional to affected depth of rainfall and swelling rate of rock mass and inversely proportional to tunnel buried depth.