Within the framework of dielectric continuum theory, we study the effect of ternary mixed crystal defect layer on the localized interface optical-phonon modes in a finite superlattice by using a transfer matrix method. In such a finite superlattice, the evolution of all the interface modes can be clearly tracked. Our numerical results show there exist two types of localized modes and their macroscopic electrostatic potentials mostly locate in the vicinity of the defect layer or surface layer, respectively. As the relative widths and nature of the structural cell or the defect layer are altered, the localized characteristics of these modes are significantly influenced. Though the conservation of the total number of localized modes is broken, the conservation of the total number of all interface modes including both the extended modes and localized modes is still kept for every value of the wavevector q‖.