The multiband co-aperture optical system with compact structure can achieve full and effective integration of multi-source intelligence information, which is one of the development direction of the optical system. Dichroic beam splitter is a vital optical component to make several systems with different bands share one aperture. The effect of the dichroic beam splitter on the multiband co-aperture optical system is analyzed by matrix optics method and primary aberration theory. The results indicate that the reflection angle of the dichroic beam splitter as a reflector changes imaging direction, and the wedge angle of the dichroic beam splitter as a transmission component increases some aberrations.