Abstract: In this paper,the stress-strain relationship of soil established from shear tests is distinguished into two kinds-strain hardening type and strain softening type. A new stress-strain model which can not only describes strain hardening stress-strain curve but also strain softening stress-strain curve is presented. The equation of tangent modulus is deduced. The paper presents the simplified form of the expression when it was employed to describe strain hardening stress-strain curve. The method of fixing the parameter and the relationship between the parameters and the confining pressure was studied. Comparing this new model with Duncan-Chang model and the strain softening model, it is found that this new model can better meet test results. The simplified form of new model is more accurately to describe strain hardening stress-strain curve and can describe more kinds of curve comparison with Duncan-Chang mode1. It is able to display variety of the stress-strain curve of soils using a changeable parameter. The unified expression provides a base to develop a general nonlinear mechanics model of soils.