A single-ion model approximated by point-charge interactions is proposed to account for the perpendicular anisotropy K(u) in Tb/Fe multilayer films with variant modulation periods. In multilayers, K(u) depends not only on the crystal field, but also on the orientation of the magnetic moments along the z direction in the period. A quasi-self-consistent method of determining the crystal-field coefficient D and the average orientation of the moments in the interface was developed during the fitting process of K(u). The crystal-field coefficient D = 2.4 X 10(-17) erg is determined. The best-fit values of K(u) and M(s) are in good agreement with experiment.