Based on the theory of nonlinear optical frequency conversion, the phase-match angle, walk-off angle, acceptance angle and effective nonlinear coefficient of difference-frequency in GaSe and ZnGeP2 crystals are calculated under different phase-matched conditions. In the theoretical simulation, the experimental parameters which were reported to generate widely tunable terahertz waves by nonlinear optical difference-frequency method are adopted. The optimum phase-matched conditions corresponding to different terahertz wave bands are summarized through comparing the whole calculated results. The calculated results provide a sound and comprehensive theoretical basis for using optical difference-frequency method to generate tunable terahertz wave in the nonlinear crystals.