Absolute test eliminates the errors of reference flats which limit the accuracy of interference measurement, so that the figure of optical surface can be measured with nanometer accuracy. We give a brief review and comparison of several mainstream absolute flatness test methods. And then the impacts of edge noise, figure grades of the flats under test, rotation angle error and rotation eccentricity, etc. on the measurement error of absolute test are analyzed respectively. The first 36 Zernike polynomial terms are used to construct the virtual flats under test. It is found that the measurement error is not sensitive to figure grades of the flats, resolution of interferogram and rotation angle error. However, it is sensitive to edge noise and rotation eccentricity. In an absolute test system, it is recommended that the alignment error of rotation axis is less than 2 pixel, and the ratio of central area is about 95%.