The accurate infrared solar magnetic field measurement system (AIMS) is a 1 m off-axis Gregorian alt-azimuth solar telescope and will be dedicated to measuring the solar magnetic field in mid-infrared. How to align the large-aperture off-axis system is a significant issue. Sub-aperture stitching with the small-aperture standard flat mirror can be applied to the alignment of the large-aperture off-axis system. However, this method is time-consuming and inefficient. We propose an alignment method based on the Zernike polynomials of the central small aperture to solve the low efficiency of sub-aperture stitching. Theoretical simulation shows that the RMS residual error of the system after using the central small-aperture alignment method will be less than 4.5 ∗ 10−6λ at 632.8 nm. Practical alignment suggests that our method can make the RMS value of full-aperture wave aberration quickly converge to 0.12λ at 632.8 nm. Compared with the sub-aperture stitching method, our method can significantly reduce the times of sub-aperture stitching and save the alignment time. © 2022 Optica Publishing Group