A light guide substrate, including a light coupling-in region with multiple first gratings, a light expansion region with multiple sub light expansion regions, and a light coupling-out region, is provided. Each sub light expansion region includes multiple second gratings. The sub light expansion regions include a first set of sub light expansion regions and a second set of sub light expansion regions. Each second grating in the first set of sub light expansion regions includes a first microstructure and a second microstructure. The light coupling-out region includes multiple third gratings. When an image light enters the light guide substrate from the light coupling-in region through the first gratings, the image light is first transmitted to the light expansion region in the light guide substrate, then transmitted to the light coupling-out region through the second gratings, and then emitted from the light coupling-out region through the third gratings.