Based on the theory of nonresonant Fresnel phase matching (FPM),tuning characteristics of second-harmonic generation (SHG) wave,the full-width at half-maximum (FWHM) acceptance bandwidth for total internal reflection angle,and the effective nonlinear coefficient of sum-frequency generation in ZnSe and GaP crystals are calculated under different polarization configurations. In theoretical calculation,the parameters,which have been reported to generate the tunable infrared waves by SHG in experiments,are adopted. The optimal quasi-phase-match conditions in ZnSe and GaP are summarized by comparing the simulated results,respectively. The calculated results show a sound theoretical basis for using FPM method to generate tunable mid-infrared wave in isotropic crystals.