The half power beam-width is one of important parameters of circular reflector antenna. The calculation method of the radiation field of the circular reflector antenna is briefly described. The radiation patterns of circular taper distribution, Gaussian distribution and polynomial distribution are calculated under different illumination taper levels. The relationship between beam width and illumination level is analyzed. By using the polynomial fitting technique, the relationships between the beam-width coefficients and the illumination levels for the taper, Gaussian and polynomial distributions are derived. Using the beam width coefficient, the half power beam width of circular reflector antenna can be estimated simply, quickly and accurately.