The deformation caused by soil-structure interaction will involve the problem of small strain stiffness. Saturation has the most significant influence on small strain stiffness in the shallow soil layer with lower overburden pressure. The change of saturation will also cause the change of soil void ratio. However, the influence of saturation and void ratio on small strain stiffness is not very clear, which needs to be further studied. Through the test of soil dehumidification and elastic wave velocity, the influence of saturation and void ratio on the small strain shear modulus Gmax and bulk modulus Kmax of soil was obtained. The results show that the variation of small strain stiffness with saturation can be divided into three stages. The increase of average mass density of soil in the boundary effect stage makes Gmax increase. The volume strain in the transition stage has a significant effect on Gmax and Kmax, and the large shrinkage deformation of clay leads to an increase in stiffness. After entering the residual stage, the intergranular stress caused by physical and chemical effects such as van der Waals force, electrostatic force, and chemical cementation further increases, increasing Gmax, Kmax, and volume change of the soil.