A diffractive optical waveguide (3, 100, 200, 300, 400, 500, 600, 700), comprising a waveguide substrate (3a, 100a, 200a, 300a, 400a, 500a, 600a, 700a) and a coupling-out grating (10", 110, 210, 310, 410, 510, 610, 710) formed on the waveguide substrate, wherein, the coupling-out grating is configured to couple at least a portion of light within the waveguide substrate propagating thereinto through total internal reflection substantially along a coupling-in direction out of the waveguide substrate by diffraction, the coupling-out grating (10", 110, 210, 310, 410, 510, 610, 710) comprises a first grating (11", 111, 211, 311, 411, 511, 611, 711), and the first grating comprises a plurality of first optical unit structures arranged in an array along a plane (x-y) and has a first grating vector, a second grating vector, and a third grating vector, wherein a direction of the first grating vector is a first direction (x), and the first direction is substantially consistent with the coupling-in direction, and directions of the second grating vector and the third grating vector, being respectively towards both sides of the first direction, form acute angles with the first direction, wherein the first optical unit structure has a first predetermined profile when viewed in the plane (x-y), and the first predetermined profile has a first vertex, a second vertex, a third vertex and a fourth vertex, arranged in sequence along the circumferential direction, wherein the first vertex and the second vertex are located at one end of the first predetermined profile in the first direction (x), the third vertex and the fourth vertex are located at the other end of the first predetermined profile in the first direction (x), and the first vertex and the third vertex are outermost vertices of the first predetermined profile in a second direction (y) perpendicular to the first direction; characterized in that: a first straight line ( l 1) extending through the first vertex and the second vertex and a second straight line ( l 2) extending through the third vertex and the fourth vertex are substantially perpendicular to the first direction (x), and a third straight line ( l 3) extending through the first vertex and the fourth vertex forms a first angle θ 1 with the first direction, a fourth straight line ( l 4) extending through the second vertex and the third vertex forms a second angle θ 2 with the first direction (x), where 15 °≦ |θ 1 | ≦ 45 °, 15 ° ≦ |θ 2 | ≦ 45 °, and the first angle θ 1 and the second angle θ 2 have a same positive sign or a same negative sign. the coupling-out grating is configured to couple at least a portion of light within the waveguide substrate propagating thereinto through total internal reflection substantially along a coupling-in direction out of the waveguide substrate by diffraction, the coupling-out grating (10", 110, 210, 310, 410, 510, 610, 710) comprises a first grating (11", 111, 211, 311, 411, 511, 611, 711), and the first grating comprises a plurality of first optical unit structures arranged in an array along a plane (x-y) and has a first grating vector, a second grating vector, and a third grating vector, wherein a direction of the first grating vector is a first direction (x), and the first direction is substantially consistent with the coupling-in direction, and directions of the second grating vector and the third grating vector, being respectively towards both sides of the first direction, form acute angles with the first direction, wherein the first optical unit structure has a first predetermined profile when viewed in the plane (x-y), and the first predetermined profile has a first vertex, a second vertex, a third vertex and a fourth vertex, arranged in sequence along the circumferential direction, wherein the first vertex and the second vertex are located at one end of the first predetermined profile in the first direction (x), the third vertex and the fourth vertex are located at the other end of the first predetermined profile in the first direction (x), and the first vertex and the third vertex are outermost vertices of the first predetermined profile in a second direction (y) perpendicular to the first direction; characterized in that: a first straight line ( l 1) extending through the first vertex and the second vertex and a second straight line ( l 2) extending through the third vertex and the fourth vertex are substantially perpendicular to the first direction (x), and a third straight line ( l 3) extending through the first vertex and the fourth vertex forms a first angle θ 1 with the first direction, a fourth straight line ( l 4) extending through the second vertex and the third vertex forms a second angle θ 2 with the first direction (x), where 15 °≦ |θ 1 | ≦ 45 °, 15 ° ≦ |θ 2 | ≦ 45 °, and the first angle θ 1 and the second angle θ 2 have a same positive sign or a same negative sign.