Null-corrector plays an important role in correcting the optical aberration. In order to correct the off-axis aberration of the large aperture off-axis paraboloid, a new aberration corrector structure with a decentered and tilted null-corrector set behind the large aperture off-axis paraboloid is present based on the feature of single off-axis paraboloid collimator. The monochrome aberration′s seidel coefficient of the off-axis paraboloid is further deduced, and the space relevance between the decentered and tilted null-corrector and the off-axis paraboloid is deduced to calculate the initial structure. The initial structure deduced from this method is proved right with an example and can correct the aberration with a smaller aperture.