The modified random-element isodisplacement (MREI) model is improved. The different second neighbor force constants are used to calculate the frequencies of optical phonons of the mixed crystals, and the first neighbor force constants can be evaluated to be a negative exponent variation with the composition x, thus the relationship between optical phonons frequencies and composition x of several AB1-x Cx style mixed crystals was calculated without any modulatory parameters. The theoretical results and experimental data meet well with each other, which gives certain guidance to future experiments.