Aperture field integration method (AFIM) is proposed and utilized to efficiently compute the field distributions of infrared (IR)/millimeter wave (MMW) beam splitter including the MMW near field distribution and IR far field distribution. A single dielectric layer beam splitter is analyzed by AFIM. Compared to the multilevel fast multipole method (MLFMM), the memory requirement and CPU time consumption are reduced drastically from 4.6 GB and 2141 seconds to 1.2 MB and 58 seconds, respectively, when the MMW near field distribution is computed. In order to meet the far field condition a scale factor is proposed to transform the focal plane into a far field plane, when the IR far field in a focal plane is computed. The IR far field calculation accuracy is better than 97% compared to the analytical solutions. The simulation results show that the AFIM as a unified method can be applied to both beam splitters’ MMW near field and IR far field engineering estimation.