Large-aperture space optical systems are important tools for observing our planet and conducting deep-space scientific research. More requirements have been put forward for large-aperture optical systems as the depth and breadth of related applications continue to increase. However, achieving the desired surface accuracy on lightweight materials for large-aperture mirror brings higher demands on relevant processing technologies, which increases the processing difficulty, cycle, and cost of large-aperture optical systems. Therefore, it is necessary to develop a new optical system technique with high tolerance for primary mirror machining errors to significantly reduce its machining accuracy requirements. This paper proposes a new optical system technique which introduces a small-aperture free-form surface into the large-aperture optical system's post optical path. By combining the freeform correction and its misalignment on the system's aberrations, the system's wavefront can be adjusted to correct the wavefront distortion caused by the primary mirror's machining errors. This reduced the machining accuracy requirements of the large-aperture primary mirror by about one order of magnitude, and high-quality imaging with a low-precision primary mirror is achieved.