A high-efficiency beam dividing system design was accomplished, which could be used in the receiving optical system of the active-passive composite image radar to divide the active beam from the passive beam. The beam dividing system could divide the active beam with wavelength of 850 nm, view field angle of 3.5°from the passive beam with wavelength of 9.7 μm and view field angle of 6.9°. The efficiency of the beam dividing system is over 98% for both active beam and passive beam. In addition, the structure stabilization and technical facility are the other advantages of the design. The beam dividing system design could share the same aperture for both active beam and passive beam which is important to achieve the miniaturization of active-passive composite image radar.