Abstract: The microstructure parameter of the expansive soil model, proposed by Alonso in 1999, is difficulty to test. After describing the expansive soil test under cyclic drying and wetting, the structure characteristics of expansive soil were analysed, and the mechanism of microstructure behavior of expansive soil was studied. The conclusion is that the strain observable at the macroscopic scale can be divided into two parts: one part is the coupled micro-and macro-structure strain, which comes from the swelling-shrinkage strain of particle assemblage at the microscope; the other is macrostructure plastic strain. Therefore, the expansive model was modified. In the modified expansive model, the former strain was calculated by macrostructure parameters and t, a micro-and macro-structure coupled parameter. The latter strain equaled to the elasto-plastic strain of low plastic unsaturated soils. Finally, an example indicates that the calculated result of the model is similar to that obtained from expansive soil test.