On the basis of the modified random element isodisplacement (MREI) model, a new model is set up. The dependence between the long-wavelength optical phonon frequencies and the composition of mixed crystals XB1-xCx belonged to Ⅲ-Ⅴ semiconductors is calculated. The second neighbor force constants are assumed to be far less than the first ones. Instead of a linear variation, the two first neighbor force constants can be evaluated to be a negative exponent variation with the composition, using the overlapped repulsive potential of the ion crystal combination. At last the calculated result of three mixed crystals, AsAl1-xGax, GaP1-xAsx and SbAl1-xGax, is found to be in good accordance to the experiments.