Provided are a vehicle-mounted display device and a display system. The vehicle-mounted display device includes: an image source, a light reflection module, and an image size adjustment module which are located on a light path; wherein the image source is configured to emit image light to the light reflection module according to an image to be displayed; the light reflection module reflects the image light at least twice, and emits same to the image size adjustment module; and the image size adjustment module is configured to adjust and emit the image light incident thereon.