The invention relates to a broadband silicon nitride waveguide coupler based on a sub-wavelength grating structure. The broadband silicon nitride waveguide coupler is formed by coupling two structurally identical silicon nitride waveguides in complete left-right and front-back symmetry, wherein for each waveguide, light is input from a separate straight waveguide at an input end, enters a coupling area after passing a bending tapered waveguide, arrives at a periodic grating structure, reaches the bending tapered waveguide and is output from the separate straight waveguide at an output end; the middle of each tapered waveguide is a taper decreased gradually from the straight waveguide width, two sides of the tapered waveguide are in equally wide sawtooth shape, and an interval structure of the sawteeth is identical to the grating structure; widths and heights of the waveguide coupler from head to tail are completely equal. Compared with the coupler in the conventional sense, the silicon nitride waveguide coupler provided with the designed grating structure can greatly expand the transmission bandwidth, is low in transmission loss, high in coupling efficiency and stable in performance and has important application prospect in 1550-nanometer optical communication wavebands.