The optical permitted parameters and angle tuning characteristics of ZnGeP2 (ZGP)-OPO were numerically simulated based on the dispersion equation of ZGP and the momentum and energy conservation in type-I and type-II. The angle tuning ranges of 50.7°～57° in accordance with wavelength tuning from 2.4 μm to 11 μm in type-I, and the wavelength tuning ranges of 2.4～11.5 μm (in-continuity 3～6 μm)with angle tuning 58°～87° in type-II have researched. The acceptance angle and walk-off angle in the process of angle tuning were also numerically simulated. Furthermore, the permitted parameters of ZGP were compared with AGS and AGSe crystal. The conclusions showed that ZGP is the optimum mid-infrared crystal.