In order to improve the efficiency of the single event effect (SEE) experiments on the 100 MeV proton cyclotron of China Institute of Atomic Energy, a binary energy degrader that can lower the initial proton energy to other values rapidly is designed for the 100 MeV protons provided by the accelerator. The energy degrader is comprised of six aluminum plates of 0.5, 1, 2, 4, 8, 16 and 32 mm at thickness, where the thickness of the latter plate is twice that of the previous one. We introduce the concept of relative thickness, which can also represent the order of the energy of the produced protons, and the state or operation of the degrader. The energy interval of 61 protons with energy above 9.69 MeV, produced by the degrader, is between 0.84 MeV and 4.09 MeV. And their energy straggling degree is less than 10%, and full width at half maximum of the scattering angle is less than 45 mrad. So the energy degrader basically meets the requirements of the proton SEE experiments. The influence of the initial proton energy accuracy of the protons directly provided by the accelerator on the residual energy after they have passed through the degrader is discussed. It is found that the lower the residual energy, the greater the influence is. In addition, the degrader is also suitable for protons in the 70－100 MeV energy range that the accelerator can directly provide, and more continuous energy can be obtained by using more plates designed with this method. The design method of the energy reducer proposed in this paper is simple and effective, and has a strong reference value.