Abstract: It is necessary to build a visco-elasto-plasitc constitutive model for unsaturated soils, because soils deform over time. Based on existed rate independent constitutive model, a rate dependent model which considers the influence of strain rate on soil behavior is developed. Previous test data indicated that the preconsolidate stress increased with the strain rate. Accordingly, the strain rate was included in the hardening equation, and thus the loading collapse yield surface was rate dependent. The one-dimensional constitutive equation of unsaturated soil was then obtained with the help of consistency condition. The model was used to compare with the existing test data. The model simulations show the model behaviors. The model was used to predicted experimental data for the degenerated cases included saturated condition and rate independent condition. Results show a good consistency.