A diffractive optical waveguide for optical pupil expansion, comprising: a waveguide substrate comprising a coupling-in region and a coupling-out region, wherein the coupling-out region includes a first coupling-out region, a second coupling-out region, and a transitional region, and the transitional region is disposed between the first coupling-out region and the second coupling-out region; a coupling-in grating disposed on or in the waveguide substrate and located in the coupling-in region and configured to couple input light into the waveguide substrate to cause the light to propagate within the waveguide substrate through total reflection; a coupling-out grating disposed on or in the waveguide substrate and located in the coupling-out region 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 disposed in the first coupling-out region, a second coupling-out grating disposed in the second coupling-out region, and a plurality of transitional coupling-out gratings (136, 137, 138) disposed in the transitional region; the first coupling-out grating includes a one-dimensional grating, the second coupling-out grating includes a two-dimensional grating, and the first coupling-out grating and the second coupling-out grating are arranged in a first direction; the transitional region comprises a plurality of sub-regions, and each sub-region is correspondingly provided with one of the transitional coupling-out gratings, wherein each of the transitional coupling-out gratings has a first vector direction, a second vector direction, and a third vector direction; the third vector direction of each of the transitional coupling-out gratings is parallel to the vector direction of the first coupling-out grating; the first vector direction and the third vector direction form a first included angle; and the second vector direction and the third vector direction form a second included angle; characterized in that in any two neighboring transitional coupling-out gratings in a direction parallel to the first direction, the first included angle corresponding to the one closer to the second coupling-out grating is greater than the first included angle corresponding to the farther one; and/or, in any two neighboring transitional coupling-out gratings in a direction parallel to the first direction, the second included angle corresponding to the one closer to the second coupling-out grating is greater than the second included angle corresponding to the farther one. a waveguide substrate comprising a coupling-in region and a coupling-out region, wherein the coupling-out region includes a first coupling-out region, a second coupling-out region, and a transitional region, and the transitional region is disposed between the first coupling-out region and the second coupling-out region; a coupling-in grating disposed on or in the waveguide substrate and located in the coupling-in region and configured to couple input light into the waveguide substrate to cause the light to propagate within the waveguide substrate through total reflection; a coupling-out grating disposed on or in the waveguide substrate and located in the coupling-out region 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 disposed in the first coupling-out region, a second coupling-out grating disposed in the second coupling-out region, and a plurality of transitional coupling-out gratings (136, 137, 138) disposed in the transitional region; the first coupling-out grating includes a one-dimensional grating, the second coupling-out grating includes a two-dimensional grating, and the first coupling-out grating and the second coupling-out grating are arranged in a first direction; the transitional region comprises a plurality of sub-regions, and each sub-region is correspondingly provided with one of the transitional coupling-out gratings, wherein each of the transitional coupling-out gratings has a first vector direction, a second vector direction, and a third vector direction; the third vector direction of each of the transitional coupling-out gratings is parallel to the vector direction of the first coupling-out grating; the first vector direction and the third vector direction form a first included angle; and the second vector direction and the third vector direction form a second included angle; characterized in that in any two neighboring transitional coupling-out gratings in a direction parallel to the first direction, the first included angle corresponding to the one closer to the second coupling-out grating is greater than the first included angle corresponding to the farther one; and/or, in any two neighboring transitional coupling-out gratings in a direction parallel to the first direction, the second included angle corresponding to the one closer to the second coupling-out grating is greater than the second included angle corresponding to the farther one.