The study of secondary optical design of extended sources is significant in the applications of high-power light-emitting diodes in the lighting field. Herein, first, the causes of degradation in the illumination effect in light distribution systems for uniform illumination under extended source conditions are analyzed. Then, a refractive index pre-compensation method is proposed. Based on this method, a light distribution lens for uniform illumination is designed for extended sources with aperture of 8 mm. Simulation results show that the length of the collapse edge of the spot significantly decreases such that the slope improves with respect to the light distribution lens, and thus, the effective illumination area on the target surface increases. Consequently, the average illuminance of the target area increases. The normalized root-mean-square of the effective illumination area further decreases, and the uniformity of illuminance improves. The illumination efficiency improves because the light energy is able to further concentrate on the effective illumination area. This method provides a new approach to realize uniform illumination for extended source.