In order to study the factors of collimation system in the terahertz scattering measurement, the collimation system in Terahertz scattering measurement system is modeled and simulated by the use of the optical design software Zemax. From the laser beam waist and divergence angle of emission laser these two aspects respectively, in 118.83 μm terahertz emission sources, the effects of emission laser on collimation properties, width and divergence angle of collimated beam are studied, which provide theoretical supports in the scattering characteristics of the actual more accurate measurements.