In sub-aperture stitching system, image quality will be affected directly by tilt and shift errors of sub-apertures. The equivalent nodal point concept of dynamic optics was applied. During alignment of sub-aperture stitching system, if the sub-aperture was tilted and shifted, the circle center of sub-aperture would be tilted and shifted consequently. The equivalent nodal point will shift would sub-apertures. The spot size variation due to sub-apertures image displacement was analyzed in detail. The effect of sub-aperture tilt and shift on image quality in stitching system was simplified with equivalent nodal point concept. Through application in structure design and tolerance analysis of sub-aperture stitching prototype, the tilt tolerance and displacement tolerance is 4 μrad and 6 μm respectively. Consequently the alignment tolerance was controlled effectly in stiching prototype.