Design method of an aperture-divided optical system used for polarization imaging systems is introduced and discussed in detail. Aperture-divided optical system is a decentered optical system composed of several coaxial subsystems. Its primary aberrations are represented based on the primary aberration theory. The initial structure parameters are calculated with the PW method and least square algorithm. A four sub-aperture optical system with effective focal length of 40 mm, F-number of 4.5 and wavelength range from 450 nm to 650 nm is optimized. The system uses an ordinary optical glass with total optical length of 120 mm. Imaging performances of the designed system approache to diffraction-limited and can meet the application requirements of the polarization imaging.