It is difficult to cope with the changes in power and polarization for partially polarized light. The light polarization will also affect the image quality, especially for imaging with large numerical aperture. A novel method of analyzing the power and the polarization state of partially polarized light is proposed in this paper. This method has a good intuitiveness and requires much smaller computation. An aplanatic lens system is simulated. Polarization effect caused is analyzed with this method. The results show that for the imaging light the intensity of TM polarized light relatively increases because of large numerical aperture, which will influence image contrast. Imaging in high refractive index medium can be used to improve this problem.