Abstract: The head mounted display (HMD) with a free-form-surface (FFS) prism owns merits of small volume, good image quality and wide FOV. The initialization structure of optical system with tilted surfaces was designed according to the parixial vector aberration theory. A FFS optical system for a practical HMD was designed by entering the initial structure paramiters to the Zemax software to optimize the designed results. The result verifies the feasibility of this method, and indicates that the method to design the FFS prism in HMD can be applied to the design of other optical system with titled suface.