Abstract: The process that Q switched tunable CO2 laser pumps nonlinear GaSe crystal was studied. Based on the collinear phase match type, the phase matching angle, effective nonlinear coefficient, walk-off angle and acceptant angle were calculated. The results are as follow: THz wave between 73.84 and 3 000 m can be generated using DFG based on GaSe crystal which has high effective nonlinear coefficient of 41-54 pm/V. The walk off angle is big, which is between 1.771 and 10.63. With the small acceptant angle 13.95-94.7 mrad, the requirement in directional property is strict. When the THz wavelength is above 500 m, the two phase matching types, oe-e and oe-o, are very similar except the effective nonlinear coefficient. In experiments, it was up to the experiment conditions to decide which type to choose. The simulation result can guide the experiments about DFG in GaSe crystal.