Herein, a vehicle-mounted head-up display system based on the reflective microprism array waveguide structure is proposed. The effects of the inclination angle, number of regions, arrangement period, and incident light width of the reflective microprism array structure on the stray light, continuity of exit pupil expansion, and uniformity of illumination are analyzed. The design method using the optimal parameters is proposed. In response to the vehicle requirements of a field of view of 10°×5° and an eyebox of 130 mm×50 mm, a specific optical waveguide design and simulation are performed. Results show that the uniformity of illumination in each field of view of the proposed system is greater than 64% and that a full-color display can be realized. Moreover, the overall volume is less than 6.5 L. The proposed structure and method are expected to be applied to the field of vehicle-mounted head-up display systems to reduce their volumes.