The conversion efficiency of terahertz (THz) wave generated in nonline ar optical rectification is obviously improved with the increase of femtosecond laser power.However,the nonlinear crystals tend to be damaged under high-power femtosecond lasers,which seriously affects the generation and output of THz wave.Therefore,it is of great significance to study the damage threshold of nonlinear crystals irr adiated by femtosecond lasers.In this paper,based on the classic two-temperature coupling model,the influence of io nization process,including electronic excitation and carrier absorption,has been taken into account,and th e prediction model of damage threshold of nonlinear terahertz crystals under femtosecond lasers has been furt her built up.The variation of the temperature field of terahertz crystals irradiated by femtosecond lasers has bee n numerically simulated by the finite-difference method,and the damage threshold of terahertz crystals has bee n estimated.On the basis,the variations of damage thresholds of LiNbO₃,ZnTe and ZnSe crystals with the puls e width of femtosecond lasers have been analyzed and compared.The results show that the forbidden band width and specific heat capacity of the crystal significantly affect the damage threshold.In addition,the damag e threshold is higher for nonlinear terahertz crystals with larger forbidden band width and heat capacity.Consequent ly,LiNbO_3crystal is superior in generating high-power THz wave due to its higher damage threshold.