A diffraction optical waveguide (100, 200), comprising a waveguide substrate (100a, 200a) and a grating structure formed on the waveguide substrate (100a, 200a), wherein, the grating structure comprises a plurality of optical unit structures arranged in an array along a plane (x-y), wherein each optical unit structure is a concave hole structure or a convex structure formed on the waveguide substrate (100a, 200a) and has a first end and a second end in a first direction (x) parallel to the plane (x-y), and a distance between the first end and the second end along the first direction is a length L of each optical unit structure; and the grating structure is configured as a coupling-out grating, the coupling-out grating couples at least a part of the light propagating thereinto along a coupling-in direction within the waveguide substrate (100a, 200a), out of the waveguide substrate by diffraction, and the coupling-in direction is substantially parallel to the first direction (x); characterized in that: each optical unit structure has a maximum width W perpendicular to the first direction (x) in a predetermined section along the first direction, where 0.3L≤W≤0.7L; in the first direction (x), a central position of the predetermined section is at a predetermined distance d from the first end, where d<0.5L, and a width of the optical unit structure in a direction perpendicular to the first direction (x) gradually decreases from the predetermined section to the first end as well as from the predetermined section to the second end, so that a centroid of a cross-section of each optical unit structure parallel to the plane (x-y) is closer to the first end relative to the second end; and the first end of each optical unit structure is located upstream along the coupling-in direction, and the second end is located downstream along the coupling-in direction. the grating structure comprises a plurality of optical unit structures arranged in an array along a plane (x-y), wherein each optical unit structure is a concave hole structure or a convex structure formed on the waveguide substrate (100a, 200a) and has a first end and a second end in a first direction (x) parallel to the plane (x-y), and a distance between the first end and the second end along the first direction is a length L of each optical unit structure; and the grating structure is configured as a coupling-out grating, the coupling-out grating couples at least a part of the light propagating thereinto along a coupling-in direction within the waveguide substrate (100a, 200a), out of the waveguide substrate by diffraction, and the coupling-in direction is substantially parallel to the first direction (x); characterized in that: each optical unit structure has a maximum width W perpendicular to the first direction (x) in a predetermined section along the first direction, where 0.3L≤W≤0.7L; in the first direction (x), a central position of the predetermined section is at a predetermined distance d from the first end, where d<0.5L, and a width of the optical unit structure in a direction perpendicular to the first direction (x) gradually decreases from the predetermined section to the first end as well as from the predetermined section to the second end, so that a centroid of a cross-section of each optical unit structure parallel to the plane (x-y) is closer to the first end relative to the second end; and the first end of each optical unit structure is located upstream along the coupling-in direction, and the second end is located downstream along the coupling-in direction. each optical unit structure has a maximum width W perpendicular to the first direction (x) in a predetermined section along the first direction, where 0.3L≤W≤0.7L; in the first direction (x), a central position of the predetermined section is at a predetermined distance d from the first end, where d<0.5L, and a width of the optical unit structure in a direction perpendicular to the first direction (x) gradually decreases from the predetermined section to the first end as well as from the predetermined section to the second end, so that a centroid of a cross-section of each optical unit structure parallel to the plane (x-y) is closer to the first end relative to the second end; and the first end of each optical unit structure is located upstream along the coupling-in direction, and the second end is located downstream along the coupling-in direction.