Abstract: The multi-field coupling problem of water, heat and salt in the composite unsaturated soils composed of unsaturated clay and unsaturated sand is studied. Based on the mass and energy conservation equations for porous media, the temperature, pore pressure, salt content and their gradients are selected as the state variables, and the state equations for the multi-field coupling problem of water, heat and salt under unsteady conditions are established. After the Laplace transformation of the state equations, the frequency-domain solution for the strongly coupled nonlinear variable coefficient differential equations is solved, and the time-domain solution is obtained by the inversion method. The numerical solution is compared with the experimental results of the self-designed tests, and the accuracy of the established mathematical model is verified. The influences of the thickness and saturated permeability coefficient of the unsaturated sand at the bottom of the composite unsaturated soils on the distribution and migration law of water and salt fields in the whole unsaturated soils under the temperature gradient are analyzed, and then the test and theoretical basis are provided for the improvement of saline soils and the construction of engineering soil barriers.