With the uprating requirements of space remote sensing, the aperture of the remote sensor is getting larger and larger. The influences of both the support of optical elements and gravity deformation on the optical system are difficult to conquer. Therefore, it is necessary to compensate the descending optical performance which is caused by the surface error of the primary mirror by means of adjusting the position parameters of the optical elements on-orbit. A large aperture coaxial three-mirror optical system is introduced. The variation of primary aberrations caused by tilting, off-centering and varying of axial position of the secondary mirror are theoretically analysed. The on-orbit compensation ability of the secondary mirror is deduced consequently. Zernike polynomials are used to simulate the surface error of the primary mirror. The spherical aberration, coma and the astigmatism of the primary mirror are compensated respectively by adjusting the positional freedoms of the secondary mirror, which are proved by both simulation results and feasibility analysis. The results show that the adjustment of the positional freedoms of the secondary mirror can compensate both the coma and some astigmatism of the primary mirror, but a little spherical aberration.