Normally, Strehl ratio, resolution, and focus depth, the parameters that are used to evaluate the performance of an optical imaging system, are always calculated by scalar diffraction method. Considering the vectorial properties of light, especially when the system numerical aperture (NA) is high, the accuracy of scalar method can no longer fulfill the requirement of analysis. The variations of properties of these parameters accompanied by using of vectorial method are discussed. In order to describe the effects of polarization clearly, assuming a system with identical polarization state within the entrance pupil, by formula deduction and numerical simulation, we conclude that due to the influence of the polarization, the maximum intensity of system diffraction spot decreases, the resolution between image plane and parallel polarized orientation of incident light drops down, then the diffraction spot loses its circular symmetry, the focus depth changes slightly, and these effects becomes more apparent with the increase of system NA. Finally, the method of calculating the off-axis images is discussed in the paper.