In order to measure a convex aspherical surface with large aperture, back null compensator test method was proposed. By introducing an auxiliary spherical surface at the back of convex aspherical surface and introducing spherical compensator in optic system, this novel test method is able to realize null test. The auxiliary spherical surface is not only essential to form an aspherical lens with convex aspherical surface, but also can compensate part of normal aberration of convex aspherical surface. Based on the third-order aberration theory, the radii of auxiliary spherical surface and optical parameter of compensator of initial system are calculated. By using commercial optical design soft the initial system is optimized and back null compensator is designed. An aspherical surface with 120 mm diameter is tested to prove the practicability of this method. As the result, the residual wavefront error is smaller than 0.007λ rms (PV 0.024λ). The precision of convex aspherical surface which is measured by this method is better than 1/40λ.