An anion negative adsorption equation in the condensed colloidal suspension with overlapping flat double layers was derived according to Gouy-Chapman theory. The electric potential at the outer Helmholtz plane (OHP), psi(delta), and the electric potential at the midplane, psi(d), were numerically solved by computer using the anion negative adsorption equation on the basis of experiments. The results showed that psi(delta) and psi(d) increase with the decrease of the distance between two clay plates, lambda, at first in the given electrolyte concentration. When lambda is smaller than 50-70 Angstrom, psi(d) remains almost unchanged while psi(delta) declines remarkably with the further decrease of lambda. The change of psi(d)/psi(delta) can explain and manifest overlapping degree of flat double layers more appropriately than psi(d) in previous works. Due to compression of the flat double layer on the clay colloid surface at increasing electrolyte concentration, the magnitude of the electrical potentials at OHP and midplane is considerably reduced at a given lambda.