A null corrector was designed to compensate 0.087 mm surface departure from the best fitting sphere for a Φ1.2 m F/1.5 paraboloidal mirror and to realize the surface error less than 0.033λ(RMS).The structure design,error analysis,manufacturing,certification and final application of the corrector were introduced in detail.The certification by Computer-Generated Hologram(CGH) in advance shows that the shape and conic errors produced by corrector are 0.012λ(RMS)and 0.006 4%,and the final null testing results give that the surface and conic errors of primary mirror are 0.027λ(RMS)and 0.030 6%,which is consistent with the tolerance analysis well.The results indicate that the design,tolerance analysis and the quality of the corrector are rational and reliable,and it can lay a solid foundation for the null test of larger primary mirror.