Abstract: The method of moment (MoM) is inefficient in near field calculation of electrically large dielectric object, so that the aperture field integration method (AFIM) is employed for evaluating near field uniformity of IR/MW beam combiner. Compared to MoM, the calculation error of AFIM was less than 10% while the calculation time was shortened from hours to a few seconds. Standard deviation of electric field distribution was chosen as the evaluation criteria, and calculation showed that, the near field uniformity of beam combiner was a negative power function of D²/λL, where D was the size of beam combiner, L was the distance of calculated field, and λ was the wave length. The negative power function was used for optimization of size and shape of beam combiner. Optimized beam combiner was validated by MoM and the calculation error was less than 10%.