The Helmet-Mounted Displays (HMDs) are becoming compact and lightweight in recent years.To meet the requirement of it a compact catadioptric optical system was designed.The prism was thinner than before by reducing the obliquity of beam splitter.Doublet and aspheric surface were introduced into the system to correct the chromatic aberration and other aberrations effectively.All the elements were agglutinated to make it compact and easy for assembly and adjustment.The designed technical specifications of the optical system are:FOV of 27°×20.4° exit pupil distance of 25 mm exit pupil diameter of 12 mm back focal length more than 5 mm Modulation Transfer Function (MTF) higher than 0.5 at the frequency of 40 lp/mm and the distortion was less than 0.5%.The results show that the optical system has a fine performance and can meet the aberration characteristics demands of eyepieces.The system can be widely used in various fields of virtual reality.