In order to investigate the differences between the dynamic response problems of quasi-saturated and saturated foundations. Based on the theory of quasi-saturated porous media, the dynamic response problem of a semi-infinite quasi-saturated soil foundation is investigated. Using the Fourier integral transform, the computational lexicon of the dynamic response of a quasi-saturated soil foundation under bar simple harmonic loading on the ground surface is established according to the Helmholtz vector decomposition principle. The effects of saturation degree and loading frequency on soil displacement, stress, and pore water pressure in the quasi-saturated foundation were analyzed. The results show that the loading frequency and the degree of saturation greatly influence the dynamic response of the quasi-saturated soil. With the increase of saturation, the surface displacement magnitude and positive stress magnitude increase, especially when Sr = 1, the surface displacement magnitude and positive stress magnitude change significantly, but the value of shear stress is not sensitive to the change of saturation. Pore water pressure increases with saturation and is most significantly affected by saturation relative to stress and displacement.