Head mounted display (HMD) as a kind of visual systems requires a large field of view (FOV), a large exit pupil distance, a small volume, and light in weight. In order to meet these requirements, a compact two-mirror structure and free form surfaces were introduced to the system. Two-mirror structure was benefit to minmize the weight and volume of the system. Free form surfaces was used to correcte the aberrations introduced by wide FOV and non-rotationally symmetry of the HMD. The abberation compensate theory and design method were also described in detail. An achromatic HMD with 50°×25° FOV, 8 mm pupil, 27 mm eye clearance was designed. The modulation transfer function (MTF) value was more than 0.4 within the entire FOV. The system could be applied to the next generation HMD technology.