To realize the mode conversion between the terahertz(THz) dielectric waveguide and the metal waveguide, we design a low-loss, high coupling efficiency tapered waveguide coupler. We have simulated and optimized the transmission efficiency of this structure by the finite-difference time-domain(FDTD) method, and the transmission loss of the fundamental mode is extremely low when the frequency is 170-220 GHz, and TM fundamental mode near lossless transmission at 190-200 GHz. This functional device can be used for coupling of integrated THz chips.