A diffractive optical waveguide, comprising a waveguide substrate and a grating structure formed on the waveguide substrate, wherein the grating structure is used to couple at least a part of light that propagates into it within the waveguide substrate along a coupling-in direction, out of the waveguide substrate through diffraction, the grating structure comprises a plurality of optical unit structures arranged in an array along a plane, and the optical unit structures are columnar structures, and have cross-sections (10A, 10A', 10B, 10B', 10C, 10C') parallel to the plane; the array comprises a plurality of rows of optical unit structures, the plurality of rows have a predetermined interval D in the first direction (x) and extend along a second direction (y) perpendicular to the first direction (x), the optical unit structures in each row are arranged at a period P, and the optical unit structures in two adjacent rows have a predetermined misalignment amount s in the second direction (y), and s=P/n, wherein 1<n ≤10; the cross-section (10A, 10A', 10B, 10B', 10C, 10C') as a whole has a shape with two small ends and a large middle part and has an upper vertex and a lower vertex in the first direction (x) and a left vertex and a right vertex in the second direction (y), and the upper vertex, the lower vertex, the left vertex, and the right vertex are respectively convex extreme points of contour of the cross-section; and a distance between the upper vertex and the lower vertex of the cross-section (10A, 10A', 10B, 10B', 10C, 10C') in the first direction (x) is length L of the cross-section, a maximum distance between the left vertex and the right vertex in the second direction (y) is the maximum width W of the cross-section, and 0.4L≤W≤0.8L; characterized in that: a first contour curve, a second contour curve, a third contour curve, and a fourth contour curve are respectively formed between the upper vertex and the left vertex, between the upper vertex and the right vertex, between the lower vertex and the left vertex, and between the lower vertex and the right vertex, the first contour curve and the second contour curve are configured such that as approaching the upper vertex, a width of the cross-section in the second direction (y) gradually decreases, and the third contour curve and the fourth contour curve are configured such that as approaching the lower vertex, a width of the cross-section in the second direction (y) gradually decreases; and the first contour curve and the second contour curve are smooth and continuous at the upper vertex and have an upper radius of curvature R 1, and the third contour curve and the fourth contour curve are smooth and continuous at the lower vertex and have a lower radius of curvature R 2, wherein R 1 ≤L/19, R 2 ≤L/19. the array comprises a plurality of rows of optical unit structures, the plurality of rows have a predetermined interval D in the first direction (x) and extend along a second direction (y) perpendicular to the first direction (x), the optical unit structures in each row are arranged at a period P, and the optical unit structures in two adjacent rows have a predetermined misalignment amount s in the second direction (y), and s=P/n, wherein 1<n ≤10; the cross-section (10A, 10A', 10B, 10B', 10C, 10C') as a whole has a shape with two small ends and a large middle part and has an upper vertex and a lower vertex in the first direction (x) and a left vertex and a right vertex in the second direction (y), and the upper vertex, the lower vertex, the left vertex, and the right vertex are respectively convex extreme points of contour of the cross-section; and a distance between the upper vertex and the lower vertex of the cross-section (10A, 10A', 10B, 10B', 10C, 10C') in the first direction (x) is length L of the cross-section, a maximum distance between the left vertex and the right vertex in the second direction (y) is the maximum width W of the cross-section, and 0.4L≤W≤0.8L; characterized in that: a first contour curve, a second contour curve, a third contour curve, and a fourth contour curve are respectively formed between the upper vertex and the left vertex, between the upper vertex and the right vertex, between the lower vertex and the left vertex, and between the lower vertex and the right vertex, the first contour curve and the second contour curve are configured such that as approaching the upper vertex, a width of the cross-section in the second direction (y) gradually decreases, and the third contour curve and the fourth contour curve are configured such that as approaching the lower vertex, a width of the cross-section in the second direction (y) gradually decreases; and the first contour curve and the second contour curve are smooth and continuous at the upper vertex and have an upper radius of curvature R 1, and the third contour curve and the fourth contour curve are smooth and continuous at the lower vertex and have a lower radius of curvature R 2, wherein R 1 ≤L/19, R 2 ≤L/19.