We explore the terahertz (THz) wave generation based on surface-emitted difference frequency generation (SEDFG) in periodically-inverted GaAs crystals. One scheme applying collinear quasi phase matching (QPM) is proposed. The wave propagation of the higher frequency pump wave is parallel to the wave propagation of the lower frequency pump wave, and both of them are perpendicular to the domain wall of the nonlinear crystal. The THz wave is emitted perpendicular to the surface of the nonlinear crystal. We calculated the grating period of periodically-inverted GaAs at the range of 100-1000μm (0.3-3THz). Our estimates illustrate that THz wave output corresponds to the power conversion efficiency of 0.015% at a frequency of 1THz. The result is shown that this scheme is efficient to obtain THz wave.