Terahertz (THz) scattering measurement is of great importance in many research areas such as THz imaging. To gain high-collimated, broad light and reduce loss of energy, off-axis parabolic mirrors are used during the measurement. However, the introduction of off-axis parabolic mirrors causes troubles in alignment. By using of optical design software Zemax, analyzing collimation system and collection system in the optical system. Through analysis of tilt angle and eccentricity of three off-axis parabolic mirrors, the maximum permissible alignment errors of off-axis parabolic mirrors in 118.83-μm THz emitter system are obtained.