The mean ionic activity coefficients of 38 single 1:1 electrolytes are correlated by a simplified nonprimitive model of mean spherical approximation (sigma(+) = sigma not equal sigma(n)) combined with statistical associating fluid theory, in which only the diameter of ions (sigma(i)) is treated as adjustable parameter. The ion-ion, ion-dipole and dipole-dipole interactions are represented by three implicit parameters, b(0), b(1) and b(2), respectively. The statistical associating fluid theory is adopted to deal with the association interaction for water molecules. The correlated accuracy with this simplified equation is acceptable. Some advantages and disadvantages of the model are discussed.