A coupler for coupling terahertz radiation into a waveguide was designed, which is the combination of a conical column and a short cylinder made of aluminum. The performance of the coupler in the cases of a broadband collimated terahertz wave and a single frequency collimated terahertz wave from free-space into an anti-resonate waveguide was simulated by theory and verified by experiment. The results confirm that the coupler is a high efficiency and broad band THz coupler. By optimizing the dimension parameters of the coupler, the amplitude coupling ratio of 71% and the amplitude concentration factor of 6.125 in the terahertz time domain spectroscopy system have been achieved.