Null testing by null lens is the most important method to test high precision aspherical surface, and the error of null lens affects the measurement accuracy of the aspherical surface directly. In testing processing, the error of null lens brings two kinds of wavefront aberration, among which one is symmetrical, the other is unsymmetrical. In the paper, a technique to correct the unsymmetrical part was described. The technique is based on expressing wavefront aberration in terms of zernike's circle polynomials. By turning the null lens on different degrees along its optic axis, different testing results were got. According to these results, wavefront aberration's unsymmetrical coefficients of zernike's circle polynomials which were introduced by the error of null lens were gained, then a corrective file to correct the last testing result can be made. Experimental result shows this technique is very effective, and it has been successfully applied to testing a hyperboloid convex surface.