Abstract: A large area of deep residual deluvial expansive soils is distributed in seasonally frozen soil regions in China. Its unique 'frost heave-thaw settlement' and 'expansion-shrinkage' characteristics induce many engineering diseases. Aiming at the problem of frost heave mechanism of unsaturated expansive soils in seasonally frozen soil regions, taking Yanji expansive soil as the object, the experimental studies on the frost heave characteristics of expansive soils are carried out, and it is proved that the expansion characteristics of the expansive soils due to water absorption have a significant influence on its frost heave characteristics. Then the freezing-swelling/shrinkage mechanism of unsaturated expansive soils considering the phase change kinetic area is put forward. Based on the multi-field coupling analysis method of frozen soils and the theory of crystallization kinetics, the coupled hydro-thermo-deformation frost heave model for unsaturated expansive soils, named FH_ ex_ Model, is established and verified. The proposed model can inverse the frost-heave deformation component and the expansion deformation component in the frost heave process of unsaturated expansive soils. In addition, according to the above researches, it is suggested that attention should be paid to the high freezing-induced expansion deformation in the initial freezing period in the expansive soil engineering site, and the deformation in the area below the stable freezing depth should not be ignored.