The anisotropic elastoplastic constitutive model of saturated soil is extended to describe the mechanical properties of the unsaturated soil by using the degree of saturation and effective stress as the state variables. Through the theoretical simulation of the existing dynamic triaxial tests of the unsaturated soils under completely undrained conditions

the correctness of the proposed constitutive model of unsaturated soils is verified. Finally

based on the proposed constitutive model

the influence of initial saturation on the dynamic mechanical behavior of unsaturated soil is discussed under the undrained condition. The results show that the void ratio of unsaturated soil will reduce under the dynamic loading

which will result in the increase of saturation. When the initial saturation is high

the unsaturated soil will be converted to saturated soil

and the liquefaction will happen. The research results obtained in this paper are of great significance to study the mechanical behavior of unsaturated soils under dynamic loads such as earthquakes.