In order to describe the stress-strain characteristics of unsaturated soils, based on the triaxial shear tests of unsaturated soils, a concept of change rate of Poisson’s ratio (that is the change rate of tangent Poisson’s ratio with the axial strain) is proposed. It is found that the tangent modulus and Poisson’s ratio rate of unsaturated soils under triaxial shearing attenuate exponentially with axial strain. Based on this law, a new nonlinear model for describing the stress-strain relationship of unsaturated soils is proposed. It can depict both the strain-hardening and strain-softening behaviors of unsaturated soils, and also it is suited to describe the stress-strain characteristics of unsaturated soils under unconsolidated undrained, consolidated drained and consolidated undrained triaxial shearing conditions. This model contains six parameters. All of these parameters have clear physical meanings, and their values can be determined easily. The triaxial shear tests in the open literature are simulated by using the new model, it is found that this model gives a satisfactory prediction of stress and strain of unsaturated soils compared with test data, thus the rationality and applicability of the proposed model are validated.