In order to solve problems involved in high-precision surface map testing of coaxial high-order aspherical surfaces, this paper proposes a null compensation testing method based on CGH. Based on this method, the separation of the diffraction order in the coaxial aspherical compensation design can be effectively realized, and the null compensation design of the mirror to be measured can also be realized. Combined with engineering examples, this paper realizes a null compensation testing design for a coaxial high-order aspherical mirror with a 260 mm aperture. The CGH design results show that the theoretical design testing residual (RMS value) can reach 0 nm based on the designed method. The practical testing of the coaxial high-order aspherical mirror is also completed. To further analyze the testing results, error analysis is carried out on the error source in the testing process, to verify the reliability and accuracy of the method.