The helmet-mounted display demands light weight, suitable size, and compact structure. Aiming at the demands, we use the off-axis catadioptric principle to design a helmet-mounted display system, and the monolithic freeform surface prism is utilized to solve the problem of small exit pupil diameter. The designed system parameters are as follows: the diameter of the exit pupil is 8 mm, the angle of field of view is 20°(H)×15°(V), distance of exit pupil is 20 mm, a 0.47 in OLED-XLTM display is selected, the display area is 9.6 mm×7.2 mm, the pixel number is 640 pixel×480 pixel, and the pixel size of 15 μm×15 μm. A single element is adopted to design the system, whose volume is less than 13 mm×25 mm×17 mm. K26R is selected. The modulation transfer function at 30 lp/mm in the full field of view is higher than 0.25. The system is designed with a single element, and realizes small volume and light weight with imaging quality taken into consideration.