According to the non-imaging optics theory, a freeform surface design method based on collimation system is proposed. With the geometrical optics as the theoretical foundation, a new type of light-emitting diode (LED) collimation system is designed based on the rotational symmetry of the central axis. The light collecting angle of the collimating system is about 236°, and the semi-luminous angle of the emitted light is less than 1°.Based on geometrical optics and the law of energy conservation, a freeform surface is designed on the exit plane of the collimating system. The ‘many-to-one’ topological relation of energy and the illumination design method of uniform rectangular spot are presented. The surface data of freeform surface is solved by Matlab software. The model of this surface is built on the basis of non-uniform rational B-splines (NURBS) surface theory by SolidWorks software. Simulation analysis is carried out by LightTools software of optical design. The simulation results show that the new collimating system can collimate the light of large angle, and the design method of the freeform surface based on the collimating system can achieve uniform light spot of rectangle.