To meet the requirement of uniform illumination, a new automatic optimization design method for LED extended source is proposed. According to point source approximation theory, a reflector for uniform illumination is established. With chips-on-board (COB) type LED source replacing point source, our approach is used to achieve uniform illumination on the target plane with optimized illumination function defined by the “scheme” language in TracePro. A Lambert emission of LED with diameter of 20 mm is used as simulation source. The result shows that our optimization method works well with uniformity (the ratio of average illumination to the maximum illumination) of 85% or more and optical coupling efficiency of 96% or more at the target plane 300 mm away from the source. This design method is easy and useful, with a wide range of applications.