Abstract: A sandy seabed is permeable, and it will cause the changes in wavelength. Based on the linear wave theory, a perturbation method proposed by Mendez is applied to solving the dispersion equation for waves over the porous bottom numerically, obtaining the wavelength under the given conditions of wave period, water depth and permeability coefficients. And then the changes in wavelength of the wave propagation over the sandy seabeds with different permeability coefficients and water depths are studied. The research results indicate that: ① the wavelength on the sandy seabed is longer than that on an impermeable bottom, and it will increase with the increase of the permeability coefficients; ② when waves propagate toward the shore, the decreasing degree of the wavelength by wave shoaling on the sandy seabed is smaller compared with the impermeable bottom, and the differences are more remarkable in a shallower water depth. Therefore, the effect of the permeable sandy bottom on the wavelength is equivalent to a variable Δh, namely the water depth of the seabed, which increases with the increase in the permeability coefficients and decrease in the water depth. As a result, the wavelength of the wave propagation over the sandy seabed with a water depth h equals that over the impermeable bottom with a water depth h+Δh.