Abstract: With the rapid development of virtual reality（VR） and augmented reality（AR）, more and more VR and AR related applications come into the views of the general public. Among various displays, the head mounted displays（HMD） play an important role due to its high immersion, real-time interactive and wide applications. This paper introduces the imaging principle of the optical system on the traditional HMD, and summarizes three kinds of novel designs of HMD, which includes the free-form optics based HMD, the waveguide based HMD and the real stereoscopic HMD. All these HMDs can solve such problems as large volume and weight, the conflicts between the accommodation and convergence of the traditional head mounted displays with different approaches. This paper also presents the subjective and objective evaluation methods of HMD as well as its future development trends.