We design a coupler for coupling parallel terahertz radiation into a terahertz anti-resonate waveguide sensor. It is the combination of a conical column and a short cylinder and made of aluminum. We simulate the coupler working in the case of a parallel terahertz wave and investigate the coupling results of both a broadband terahertz wave and a single frequency terahertz wave from free-space into an anti-resonate waveguide sensor. The results indicate that the transmission spectrum can be described as a sum of the transmission associated with the conical column and the cylinder part. We optimize the best dimension parameter of the coupler, with which we get the coupling ratio of 86.3% and the terahertz intensity concentration factor of 6.9.