This paper presents a calculation method of the mode coefficient (amplitude) on Vlasov radiator aperture by Method of Moments (MoM), and the reflective characteristics of bevel-cut radiator are studied through this method. The relations of the reflectivity to bevel-cut angle and diameter-wavelength ratio are obtained, which indicate that the reflectivity of the radiator is very small. The results are helpful for the design of the radiator.