The binocular see-through optical Head-mounted Display (HMD) is developed by using discrete elements，and the key problems in the HMD are researched. Using polarization beam splitter as combiner, it is found that the brightness of HMD and outside environment has cosine variation with the polarizer azimuth through theoretical analysis. Then, the image of HMD can mix together with outside environment excellently by adjusting the azimuth angle of polarizer. The variations of HMD vision effect which change with the rotation of polarizer are investigated, and the results indicate that the effect is optimal when the binocular overlapped range is 33~50% of the monocular field of view. At last the stereoscopic display of HMD is realized by the stereo image pairs which are made according to the parallax theory.