Reflective zoom system is widely used in the design of large size, wide spectral, high resolution system due to its great superiority in compacting size, system weight, aperture size, free chromatic aberration and thermo-stability. But for coaxial system, its disadvantage of obstruction renders the FOV (field of view) and light utilization rate unsatisfactory. Thus, to make the secondary and tertiary mirror off-axial is a good choice for optical designers. However, there are two problems in the alignment of off-axis zoom optical system. First, the Seidel aberration theory is not applicable for a system without rotational symmetry. Second, it is hard to control the misalignment status when zoomed. To solve this problem, the vector aberration theory is selected to analyze the off-axis three-mirror zoom system. When small perturbation is applied to the system, coincident with the alignment in reality, the residual aberration varies along with the movement of secondary and tertiary mirror. As the result, aberration character of misalignment three-mirror zoom system is provided, which offers guidance for misalignment determination and makes sure of the normal operation of the zoom system. This paper makes description of our experiment on an off-axis three-mirror zoom system and furnishes the figure of sensitivity in different zoom position. The conclusion may provide a reference to the vector aberration theory study on off-axis three-mirror zoom system and computer aided alignment.