Abstract: An anion negative adsorption equation in the clay colloid suspension of overlapping flat double layer has been derived according to the electric double layer theory of Gouy-Chapman. The electric potential at the Outer Helmhotz Plane(OHP), ψ_δ, and electric potential at the midplane, ψ_d, have been solved by FACOM M-34-OS computer using the anion negative adsorption equation on the basis of experiment. The results are concluded as following:(1) The values of ψ_δ and ψ_d increase with the decrease of distance λ (Fig.4 and Fig.5). (2) The change of ψ_d/ψ_δ with distance λ can explain the overlapping degree of flat double layer mor appropriate than that of ψ_d in previous work (Fig.6). (3) Due to compression of the flat double layers at increasing electrolyte concentrations, the values of ψ_δ and ψ_d are considerably reduced at a given distance λ.