The method of compensator, a common and important means, which could test figure error of aspheric mirror, is applied more frequently at home and abroad. And alignment of every optical element could introduce primary aberrations, which will influence the final test result. This paper expatiated the test process with compensator, which gave an illustration of 1 meter aspheric mirror, and analyzed some corresponding primary aberrations that were brought by alignment, and enumerated the mistake of test process. Then, corresponding method of judging and elimination was educed. Finally, according to the comparison of the figure error test ofФ 300mm, Ф300 mm andФ1m aspheric, it was proved that the mistake would distort the result of measurement, which validated the reliability of the method of judging and elimination. And with the correct method, the figure error of final test result is 0.038λ.