To resolve the contradictions between wide Field of View(FOV),large exit pupil and compact，lightweight,a hybrid refractive/diffractive optical system using Free Form Surfaces(FFSs) for a Head Mount Display(HMD) was designed.The dispersion characteristics of diffractive elements were used to correct the chromatic aberration,and the appropriate types of FFS were selected to correct the off-axial aberrations(astigmatism，coma and distortion). Furthermore,the FFS prism was made of plastic materials to form an optical structure without rotational symmetry to achieve compact and easy to align functions.Obtained optical system can offer the eye relief of 17.4 mm,exit pupil of 4 mm and the FOV of 55°.Moreover,the RMS diameters of spot diagram are less than 30 μm,the MTF are higher than 0.1 at the spatial frequency of 30 lp/mm across the entire visual field,and the maximum distortion is less than 10%. Experimental results indicate that the optical system has perfect performance and can meet the demands of eyepieces for aberration characteristics.