Based on non-imaging optics, a secondary optical lens is designed for indoor illumination of high-power LED which could illuminate uniformly in a large divergence angle. The mapping of LED onto the target plane is constructed by grid partition method, based on which the general equations of freeform surfaces are derived. The surface profiles are obtained by solving the equations using difference method. Using the profile data, the lens model is established with three-dimensional (3D) software. The optical system is simulated with ray-tracing software. This lens is fit for uniform illumination with the target divergence angle between 40° and 140°. A lens with 120° divergence angle lens is designed and simulated. The results show that it can keep the uniformity above 0.9 and the efficiency above 92% when the ratio of the lens aperture to the diameter of LED emitting surface is not less than 10, and the uniformity is not effected by the distance between the LED and the target plane. This design provides an effective way for the miniaturization and simplification of the indoor illumination system of high-power LED.