Abstract: In order to depict the softening characteristics of soft rock effectively,firstly,a formula for the unified strength theory taking the compression as positive is established.This theory considers the strength difference of tension and compression and the mediate principal stress effect.So it is suitable for geotechnical materials.Secondly,the limit line in π plane of this theory is derived.Thirdly,a three-dimensional unified elastic viscoplastic softening constitutive model is derived based on the limit line in π plane and the model of Yin-Graham.Parameters of the model are determined with the relevant triaxial tests on a sort of diatom soft rock which is distributed in Nodo Peninsula in Japan,belonging to the late tertiary Miocene,and formed of debris diatom,clay and volcanic ash by depositing,and usually distributed in inshore regions.And finally,numerical simulation is done.The simulated results of the three-dimensional unified elastic viscoplastic softening constitutive model are very close to the experimental results,proving that the model can be used to accurately simulate the softening behaviors of soft rock.