As society continues to progress, automobiles have become a common sight in households worldwide. However, the increase in the number of vehicles on the road has led to a series of traffic safety issues. As a result, automotive safety has become one of the most popular research topics worldwide. Among them, Head-Up Display (HUD) systems can be applied to cars, serving as an assistant to driving and improving driving safety. Through the use of a head-up display optical system, drivers can see virtual images that display important driving information and road conditions in the direction of their line of sight, eliminating the need to look down at instrument panel information and navigation information. This reduces driver distraction and improves driving safety. As HUD technology continues to advance and develop, it will gradually integrate with autonomous driving systems, becoming a standard feature in future high-tech vehicles. Nowadays, the technology of automotive HUD on the market is gradually maturing, mainly including Windshield HUD (W-HUD) and Augmented Reality HUD (AR-HUD). However, the current car-mounted HUD can only display virtual images on a single projection distance. When the driver changes the observation due to changes in vehicle speed, the visual focus distance needs to be adjusted to adapt to the HUD projection distance. Long-term and high-frequency adjustments can lead to eye fatigue for the driver. To address this issue, this paper proposes the design of a car-mounted binocular parallax optical system. Based on the current research status, a method of using a single DLP micro-projection machine to separate two images and generate two Projection Generating Units (PGUs) to display binocular virtual images for AR-HUD is proposed. According to the initial architecture of the HUD optical system, which is a coaxial reflection optical system, an off-axis reflection vehicle-mounted parallax display optical system is constructed by using an off-axis rotating reflection mirror. Then, by changing the object distance of the PGUs to the primary reflection mirror, the two free-form reflection mirrors are combined to construct an off-axis reflection binocular parallax vehicle-mounted display optical system, thereby ensuring the projection of two virtual images at different distances. The optical system uses a 5.74 inch DLP micro projector with an eyebox size of 130 mm×50 mm, a far projection field of view of 10°×3° and a near projection field of view of 4°×1°, and a virtual image projection distance of 3.8 m and 7.5 m. Image quality evaluation is analyzed and explained from the aspects of the point cloud diagram, MTF curve diagram, grid distortion, illumination curve diagram, and image simulation. The key to the optical system is tolerance analysis, which helps to determine the allowable limits of different manufacturing and alignment errors. Manufacturing and calibration errors can be reduced to an appropriate level without degrading the image quality of the optical system. If the tolerance requirements are too strict, it may increase the difficulty in processing and arranging the optical components and may also lead to unstable imaging quality during actual use. By designing the tolerance range of the curvature radius of the lens and the windshield usage area, the distance between lenses and the windshield usage area, the X-axis offset lens and the windshield usage area, the Y-axis offset lens and the windshield usage area, the axial rotation of the lens and the windshield usage area, and the axial rotation of the lens and the windshield usage area, the statistical results of the optical transfer function of the system's virtual image plane were obtained using Monte Carlo method with 1 000 randomly selected samples. It can be seen that at a spatial cutoff frequency of 4.42 1p/mm, more than 90% of the MTF of the optical system is greater than 0.63, indicating that the tolerance distribution of the system is reasonable, the tolerance capability is strong, and the design results are ideal. The design result of the dual-path AR-HUD optical system in this paper shows that, through software control, the near projection virtual image plane and the far projection virtual image plane can be displayed simultaneously or in a time-sharing manner, which improves the driver's visual experience.