Terahertz (THz) wavelength lies in between millimeter waves and infrared waves in the electromagnetic spectrum. The existing optical waveguide and microwave millimeter waves waveguide technologies can be applied to the THz band. Because of the strong absorption of THz waves by water vapour and the limitation of manufacturing processes, THz devices were mainly planar structures and rectangular waveguides were commonly used for THz source and transmission. Therefore, the conversion structure between rectangular waveguides and coplanar waveguides has plays an indispensable role in determining the performance of components and systems. In recent research, the ridge waveguide has been used for impedance matching and electromagnetic field mode conversion to accomplish the high-efficiency coupling between THz wave rectangular waveguides and the coplanar waveguides. According to the simulation of CST microwave studio, the results show that the transmission coefficient (S₂₁) of the conversion structure is higher than -3 dB in 0.2-0.4 THz frequency band, which show that the ridge waveguide converts the THz electromagnetic field with high efficiency. The performance of the conversion structure breaks the restriction of frequency which is below 0.2 THz before. This designed structure can be used in various fields, such as terahertz molecular detection and terahertz communication, and it also provides a feasible solution for terahertz mode conversion above 0.2 THz.