A terahertz cross-guide waveguide directional coupler with unsymmetrical crossed slots is proposed in this paper. By adjusting the basic structure of rectangular waveguide and optimizing the parameters of the crossed slots, this waveguide directional coupler realizes a stable coupling flatness, a good directivity and the working bandwidth within the WR4 standard waveguide. The simulation results present good coupling flatness value from -19.9dB~-22.0dB, the isolation value is greater than 32 dB, and the relative working bandwidth is 40%.