A MWIR/SWIR polarization imaging optical system is presented, and a novel common-aperture optical system for MWIR/SWIR polarization imager is designed. The MWIR optical system is composed by an all-reflective afocal system, a beamsplitter, a secondary telescope, an infrared polarizer, and an infrared objective. For controlling the apertures of the first mirror, the two face reflective mirror, the beamsplitter and the infrared polarizer, a triple imaging optical system is designed. By the beamsplitter the light is divided into the MWIR (3～5 μm) waveband and the SWIR (1～2.5 μm) waveband, they are converged on two detectors respectively by two objectives, the dual wave-band optical system realized common aperture. The imaging quality is perfect, it can be satisfied to the imager, and the design result is provided.