Based on the rigorous full vector diffraction theory, the influence of misalignment on the beam focusing characteristics of large aperture off-axis parabolic mirror (OAP) are analyzed in detail. The results show that the shape of the diffraction focal spot does not change after the translation or revolving of OAP around the z″ axis, but the center position of the focal spot shifts. The quantitative relationship between OAP’s three-dimensional translation and rotation deviation around z″ axis is obtained by using Rayleigh criterion. When OAP’s rotation deviation around x″ or y″ axis occurs, the appearance of astigmatism and coma will cause the shape of diffractive focal spot to change, and the peak intensity of the focal spot is greatly reduced. The relationship between the maximum rotation deviation angle and OAP off-axis parameters and beam parameters is discussed in detail. Therefore, understanding the influence mechanism and variation relationship of misalignment on beam focusing characteristics can provide a reliable theoretical basis for precise adjustment of OAP.