An optimal design method of elliptical beam Cassegrain antenna based on differential evolution with global and local neighborhoods algorithm is proposed to achieve high efficiency, low sidelobe, and low profile at Ku/Ka dual band. A rapid shaping method of elliptical beam antenna is given by applying the algorithm of steepest descent. Constraining of the first sidelobe level, antenna efficiency is optimized with only 17 variables, where aperture field distribution functions in the major and minor axis planes are parameterized by Non-uniform rational B-spline curves, and the transition function is expressed with a sinusoidal function. A prototype of optimized antenna with an aperture of 630mm×1150mm is fabricated and measured. The measured results show that the total efficiency is greater than 54.5%, and the first sidelobe levels are lower than -14.2dB across the Ku/Ka dual-band, which agree well with simulated ones. It is verified that the optimal design method is very effective.