A diffractive optical waveguide for optical pupil expansion, comprising: a waveguide substrate; a coupling-in grating disposed on or in the waveguide substrate and configured to couple input light into the waveguide substrate to cause light to propagate within the waveguide substrate through total reflection; a coupling-out grating disposed on or in the waveguide substrate and configured to couple out at least a portion of light propagating therein from the waveguide substrate by diffraction, wherein, the coupling-out grating comprises a first coupling-out grating, a second coupling-out grating and a third coupling-out grating, all of which are arranged successively in a first direction; the second coupling-out grating is disposed between the first coupling-out grating and the third coupling-out grating, and comprises a two-dimensional grating; the second coupling-out grating is corresponding to a region that includes a first edge and a second edge spaced in a second direction orthogonal to the first direction, and wherein the first edge is closer to the coupling-in grating than the second edge; light output by the coupling-in grating is incident on the second coupling-out grating through the waveguide substrate, and is diffracted by the second coupling-out grating into a plurality of beam splits, the plurality of beam splits include a first beam split propagating within the region of the second coupling-out grating, a second beam split propagating toward the first coupling-out grating or the third coupling-out grating, and a third beam split propagating in the same direction as the propagation direction of light output by the coupling-in grating, and the third beam split is propagable to the first coupling-out grating or the third coupling-out grating, characterized in that a first width of the first edge in the first direction is greater than a second width of the second edge in the first direction. a waveguide substrate; a coupling-in grating disposed on or in the waveguide substrate and configured to couple input light into the waveguide substrate to cause light to propagate within the waveguide substrate through total reflection; a coupling-out grating disposed on or in the waveguide substrate and configured to couple out at least a portion of light propagating therein from the waveguide substrate by diffraction, wherein, the coupling-out grating comprises a first coupling-out grating, a second coupling-out grating and a third coupling-out grating, all of which are arranged successively in a first direction; the second coupling-out grating is disposed between the first coupling-out grating and the third coupling-out grating, and comprises a two-dimensional grating; the second coupling-out grating is corresponding to a region that includes a first edge and a second edge spaced in a second direction orthogonal to the first direction, and wherein the first edge is closer to the coupling-in grating than the second edge; light output by the coupling-in grating is incident on the second coupling-out grating through the waveguide substrate, and is diffracted by the second coupling-out grating into a plurality of beam splits, the plurality of beam splits include a first beam split propagating within the region of the second coupling-out grating, a second beam split propagating toward the first coupling-out grating or the third coupling-out grating, and a third beam split propagating in the same direction as the propagation direction of light output by the coupling-in grating, and the third beam split is propagable to the first coupling-out grating or the third coupling-out grating, characterized in that a first width of the first edge in the first direction is greater than a second width of the second edge in the first direction.