The present disclosure provides in some embodiments a vehicle-mounted head-up display system, including a projector, a diffuser, and an imaging plate. The projector is configured to generate projection light beams in accordance with a to-be-projected image, so as to form a real image of the to-be-projected image at a light-entering surface of the diffuser. The diffuser is configured to expand the light beams and transmit the expanded light beams to the imaging plate. The imaging plate is configured to converge the expanded light beams into an imaging light beam, and transmit the imaging light beam to an interior of a vehicle through a front windshield of the vehicle, so as to form a virtual image of the to-be-projected image on a reverse extension line of the imaging light beam outside the vehicle.