Based on the free electron model, a self-consistent approach is adopted to calculate the Fermi energy and its variation with the layer thickness and the magnetlization alignment in magnetic multilayers . Then we study the effect of the thickness of magnetic and non-magnetic layers, especially of electron density on the oscillation period of interlayer exchange coupling in such multilayer. A comparison of our theoretical results with those obtained by using the grand potential method is made, we find that the oscillation period is strongly dependent on the electron density. When the electron density in magnetic layer is different from that in spacer, the period will change as changing the thickness ratio of spacer to magnetic layer. This new result agree with the latest experimental data. In addition, our calculation also shows that the coupling strength is mainly determined by the polarization force experienced by electrons, while the band mismatch does influence the phase and even the period of the coupling oscillation.