In order to resolve the contradictions between large exit pupil, wide field of view (FOV) and lightweight, small size, a helmet mounted displays (HMDs) system is designed, using freeform surface described by radial basis function. The principle that radial basis function describes freeform surface is introduced, and the approaches for aberration correction are analyzed. A new way is tried to determine a starting point of optimization quickly while designing. At last, the imaging quality of the optical system is analyzed. A HMD with 45°×32°, 15 mm pupil, 50 mm eye relief is designed. The modulation transfer function (MTF) value is more than 0.6 within the entire FOV at the Nyquist frequency. The magnitude of maximum distortion occurs at (-22.5°, 16°) in the field and it is measured in simulation to be -1.54%. The size of the system is 56 mm×128 mm and the weight is 136 g. The design results show that the optical system of HMDs has low aberration, and it can provide user clear symbol or video images. This HMDs system, which has good image quality, small volume, and light weight, can be applied to the next generation avionic HMD technology.