Based on the 3rd vector wavefront aberration theory of axial symmetry optical system, the vector aberration expansions of off-axis optical system was obtained by pupil zoomed factorial and pupil decenter vector in this paper, . It is known through the analysis that the off-axis optical system′s aberration are still composed of 3rd spherical aberration 3rd coma and 3rd astigmatism. Due to the impact of pupil zoomed factorial, the decrease of the off-axis optical system′s aberration coefficients were different proportion. The higher-order pupil aberration of coaxial system will induce low-order pupil aberration into off-axis mirror system. For example, if the coaxial system is not corrected for 3rd spherical aberration, it will introduce not only the spherical but also the constant coma and astigmatism into off-axis system. The aberration of off-axis system will not be asymmetry by the field center because the induced low-order aberration, and there maybe have an aberration zero point outside the center field of the view.