Abstract: To enhance the generation efficiency of THz wave, the interaction between femto-second laser pulse and GaAs crystal was studied. Firstly, the phase matching in bulk GaAs crystal between femto-second laser pulse and the generated terahertz wave was studied, and the result showed there was no intersection point between the refractive curve of pump laser and that of the THz wave, which indicated phase matching could not be achieved in this structure. Four waveguide structures then were proposed, and the refractive index curves in 0.1~6 THz were calculated based on related theory of waveguide, and the transmission length of the THz wave in the waveguide was also analyzed via the parameters of absorbance and dispersion. The results illustrate the phase matching is enhanced in the waveguide structures, and the generation efficiency of THz wave is supposed to get a promotion. The work serves the theory evidence for terahertz generation based on the interaction between femto-second laser and GaAs crystal.