In order to make the best of CO2 laser difference frequency for terahertz(THz) generation, characteristics of three kinds of nonlinear crystals are studied. Firstly, the phase matching parameters of GaAs, ZnGeP2 and GaSe are computed, performances and advantages of each crystal are compared by combining with optical parameters and damage threshold. And then, influencing factors such as phase-mismatching and crystal absorption on efficiency of THz generation are analyzed. Tunable range, efficiency and experimental operating characteristics of THz radiation generated by three kinds of crystals are gained. All the results show that it’s critical to choose crystal with low absorption coefficient.