In order to meet the requirements of collimating illumination for high-brightness light-emitting diode (LED) and overcome the shortcomings of traditional LED collimator′s large aspect ratio and heavy structure, a novel collimator with small aspect ratio, compact structure and high collimation is presented. The design of collimator is based on the assumption that the LED source is ideal point source. The rays are reflected twice in the collimator. According to the theory of the constant optical path length and the refraction law in geometric optics, the profiles of collimator′s surface are obtained through numerical solution of the constant optical path length equations, and the aspect ratio is 0.1. Using the optical software TracePro, the collimator system is traced and simulated. The results show that the divergence angle (full angular width of the radiation pattern at the half intensity) is just 5.4° for a point source with the diameter of exit aperture is 50 mm; but it changes to 6° for an actual high brightness white LED, and the irradiance uniformity and the optical efficiency within a diameter of spot 60 mm are 92% and 52.11% at 1 m respectively, which is suitable for the illuminating area of light brighteness LED. The design realizes the objectives of compact structure and high collimation, and the tolerances meet the requirements of alignment, which provides an effective way for realizing the illuminating system′s miniaturization and simplification.