The head-up display (HUD) system can help drivers to check the speed, navigation, and other related information conveniently, shorten the visual blind spot, thus reduce traffic accidents, and improve the driving experience. In order to speed up the popularization of the vehicle-mounted HUDs, a mini-projector system based on the digital micro-mirror device (DMD) is proposed, which focuses on the reflective optical system conforming to the vehicle structures, and the simulation model is provided. The image enters into human eyes through multiple reflections by the projection screen, thus forming a virtual image in front of the windshield. The design result shows the virtual image distance can reach 2.5 m so that the driver can view the image without changing sight distance. The horizontal and vertical viewing ranges reach 140 mm and 60 mm, respectively. The optical transfer functions at different positions are all close to the diffraction limit, which indicates the clear image and good projection effect. The horizontal and vertical fields of view are 12° and 6.75°, respectively, which can provide sufficient display area without disturbing the drivers' visual field.