By combining the advantages of Offner refractive compensator and Maksutov reflective compensator, aiming at the testing of concave aspheric surface with large aperture or large relative aperture, a novel null compensator named catadioptric compensator was proposed.This method uses double lens and single mirror to test concave aspheric surface, the compensator is set between the mirror under test and its curvature center. Based on third-order aberration theory, the calculating formulas of initial structure were derivated. By compensator design for a concave aspheric suface, the princple of the proposed method was verified. The aperture of the asperic surace is 1 000 mm, the vertex radius of curvature of the aspherical surface is 4 000 mm, the conicoid coefficien is 1.05, the inner hole diameter is 200 mm, the peak valley value of the residual wavefront error of the optimized system is 0.004 2λ. The result shows that the axial testing size of this compensator is short and the compensating ablility is strong. If central obscuration is allowed, it is available to test concave aspheric surface with large aperture or large relative aperture.