In this study，the relationship of the soil water characteristic curve between the air intake value and the residual water content is analyzed and used, the microwave drying kinetic model of expansive soil was established, based on soil thermodynamics and uniform dehumidification characteristics in the process of microwave drying. The logarithmic relationship between water content of expansive soil and microwave drying time was revealed. And the experimental study on the mixture of kaolin and calcium montmorillonite was carried out. The results showed that the microwave drying process of expansive soil can been described by the microwave drying kinetic model. And the obvious regularity can been presented by the characteristic parameters of microwave drying curve and specific surface area of expansive soil.