Abstract: In order to achieve the best expanding effeciency of the diffuser and to use most protons to radiate the French Test Object, the material is chosed from three aspects. They are MCS (multiple Coulomb scattering) angle, energy loss and penetration rate for proton per unit areal mass. The analyzed results of these aspects show that the bigger the atomic number is, the higher the efficiency is. The material is determined to be tungsten considering cost and applicability. The thickness of the diffuser is designed according to the demands of uniform proton flux distribution and small noise. As a result, the diffuser thicknesses are 3.1 cm without collimator and 2.4 cm with 3.0 mrad collimator.