Zero-power correctors are often used to balance the aberrations caused by other components. A two-sphere system with a two-lens zero-power corrector placed before the primary mirror is discussed. According to third-order aberration theory,the conditions of the absence of aberrations are analyzed,and the influences of different system parameters to the remaining high order aberrations of the system are investigated. The results show that the bigger the relative aperture of the primary mirror,the bigger the remaining high order aberrations;the structure of the corrector with positive lens in front of the negative one is better than otherwise;and a suitable single lens' power may make the system the lowest aberrations.