Based on the three-dimension rheological model for unsaturated soil the constitutive equations are deduced by Laplace transformation. Then the time-dependent analytic expressions of vertical strain and lateral total stress in confined compression situation are also obtained. The modified confined rheological test has been carried out to get time-varying data of vertical strain and lateral total stress. Thus all the model parameters in the constitutive model can be calculated by the methods of curve fitting and formula inversion. At last, the method of the parameters solving is proved reasonable by an example.