Abstract: Based on Deere and Miller's classification of rock,the soft rock can be classified into two kinds,namely plastic and weakly elastic. Accordingly this paper presents two types of compression extrusion,i.e.,fully plastic and plastic-semi elastic. Differential equations were established to calculate deformation of extrusion. Formulas were then derived from these equations to estimate the minimum thickness of the strata for occurrence of compression extrusion,thickness of the strata for compression deformation and failure,stress distribution within the surrounding rock near the tunnel face,volume of extruded rocks and length of extruded rocks. The conditions for extrusion and their relationship with physical and mechanical parameters of rock as well as their variations were analyzed using those formulas. The estimated minimum thicknesses of overlying strata for several tunnels match closely with the observed data,which proves the applicability of presented method in engineering practices.