To reflect the actual state of the unsaturated three-phase medium of the subgrade, the subgrade is regarded as an unsaturated porous medium with incompressible solid and liquid phases, and a compressible gas phase．Based on the representation method of unsaturated porous media, considering the effects of inertia, viscosity, and mechanical coupling, a dimensionless one-dimensional wave equation for the unsaturated subgrade is established．Taking a type of non-homogeneous boundary as an example, the problem is transformed into an initial value problem of a second-order ordinary differential equation through boundary homogenization and the method of characteristic functions, and the state-space method is used to obtain the exact solution of the one-dimensional transient response of a single-layer unsaturated subgrade．The correctness of the results is verified through numerical examples．The characteristics of the one-dimensional transient response of the single-layer unsaturated subgrade under step and sinusoidal loads are analyzed．The results show that, unlike compressible saturated and unsaturated soils, there is only one type of compression wave in the incompressible unsaturated subgrade．As the liquid phase saturation increases, the wave speed and vibration amplitude of the compression wave decrease．The conclusion can provide a reference for the optimal design and disaster prevention of unsaturated subgrades under complex environmental loads．