The head-mounted display used for virtual reality demands an optical system with large field of view and exit pupil diameter, which has the characteristics of less weight and small total length to satisfy these requirements. This article presents a design method of the head-mounted display (HMD) optical system based on the primary aberration theory. Through this method, a two- piece optical system by utilizing polymethyl methacrylate (PMMA) and polycarbonate (PC) is designed. The exit pupil diameter of the proposed system is 8 mm and its field of view is 70°. The total length is less than 70 mm, and the weight of the system is less than 30 g. The full-field relative illumination is greater than 0.4. Its axis and off-axis aberrations have been effectively corrected. The root mean square (RMS) radius of the marginal field is about 70 mm, modulation transfer function (MTF) curves have a uniform distribution, while the MTF values of the central field and the marginal field are about 0.6 and 0.4 at 8 cycle/mm, respectively. The maximum distortion of the system is less than 2%. The manufactured optical system has satisfactory image quantities according to the quantity inspection by the standard resolution panel.