A feedback optimization design method based on surface energy mapping is proposed herein. By pre-calculating the energy distribution on the target surface, the target surface mesh unit is redistributed to build a new free-form surface lens profile. A series of lenses is designed using this method, and the simulation results show that when the ratio of the target surface radius to height (DHR) is equal to 2 and ratio of the lens height to the LED width changes from 1∶1 to 1.5∶1, the nonuniformity is reduced by 8.97%-11.95%. Moreover, when the ratio of lens height to LED width is 1.5∶1 and DHR changes from 1.8 to 2.2, the nonuniformity is reduced by 7.60%-16.49%. Thus, the proposed method can effectively improve the illumination under an extended light source. The design process does not depend on the simulation software utilized, and it is simple and easy. The number of iterations is small.