Abstract：Based on unified strength theory，considering Lode parameter，unified strength parameters of materials are proposed，which change with Lode parameter and the intermediate principal stress coefficient b，and planar style strength criterion of unified strength theory which can consider the intermediate principal stress is obtained. Assuming distribution statistical probability of micro-units strength，statistical damage variable is defined；and the statistical damage constitutive equation is proposed under triaxial stress state based on the unified strength theory. The proposed constitutive equation is verified by triaxial experiment of soft rock，the comparison between experimental results and theoretical results is processed；and the results show that the proposed constitutive model can predicted the relationship between stress and strain of soft rock，especially the characteristic of strain softening. Furthermore，on the basis of the unified strength theory and the mobilized internal frictional angle of the unified strength parameter，effects of Lode parameter and the intermediate principal stress coefficient b on deviatoric stress-strain relationship are investigated theoretically；and results show that the deviatoric stress-strain curves rise to a certain value with increasing Lode parameter，and then decrease with Lode parameter；and the deviatoric stress-strain curves rise with increasing intermediate principal stress coefficient b.
