In order to reduce the weight and volume of the head-mounted display (HMD), a planar waveguide HMD is designed based on semi-transparent film array. Two orthogonally placed semi-transparent film arrays are used to expand the eyepiece′s exit pupil, as well as to make the HMD compact and light in weight. By analyzing the structure of planar optical waveguide, an optical system model is established. Then, imaging quality and irradiance distribution of the system is discussed. This system has a gross weight of about 36 g, a field of view (FOV) of 15°×20° and an exit pupil of 7 mm×12 mm. The distortion is less than 0.13%. The root mean square (RMS) spot diameter is less than 3.75 μm. The RMS wave aberration is less than 0.045λ. The modulation transfer function (MTF) is higher than 0.58 at the spatial frequency of 40 lp/mm across the entire FOV. The results indicate that planar waveguide of semi-transparent film array can enlarge eyepiece′s exit pupil effectively. This system satisfies the requirements of the HMD optical system.