The development status of the infrared polarization imaging system in China and foreign countries is analyzed. According to the application needs of medium wave infrared (MWIR) polarization imaging, a MWIR polarization co-aperture imaging system is proposed. The system is composed of the front co-aperture and the rear sub-aperture imaging systems. The rear sub-aperture imaging system includes four polarization channels which adopts different polarization components, thus full polarization imaging is realized in the system. According to the used detectors, the analysis and design of the system is completed. After the design, the modulation transfer function (MTF) of the system is more than 0.3@17 lp/mm. The maximum root mean square (RMS) spot diameter is less than the size of the detector pixel and a single pixel energy concentration rate is greater than 80%.