In order to overcome the shortcoming involved with the current theory, the new theory model of lattice vibration in Ax B1-x Cy D1-y quarternary mixed crystals was described. Using the Clausius-Mossotti formula for simplifying the motion equations, and Born-Huang equations for eliminating microscopic variables, the relationship between optical phonon and elemental concentrations (x,y) of mixed crystals was obtained. The advantage of the proposed theory model is that the optical phonon character of Ax B1-x Cy D1-y quarternary mixed crystals can be solved only by several parameters of the binary mixed crystals such as AC, AD, BC and BD. The parameters include TO mode frequency, LO mode frequency, static dielectric constant and optical dielectric constant. Taking the boundary values, the results are corresponding to ternary mixed crystals and binary mixed crystals. The theoretical results and experimental data meet well with each other, which give certain guidance to future experiments.