Absolute calibration is an important method to improve accuracy in high precise surface figure testing. But for flat absolute calibration, the power of the flat cannot be calibrated in both the classic three-flat and the rotation- shifting absolute calibration. Using liquid surface absolute calibration can give flat surface figure including the power term, but the liquid surface is easily affected by environments and the repeatability is not good enough to get high accurate results. In order to solve the problem, liquid surface is used only to calibrate the power term and the other Zernike terms are calibrated by the rotation - shifting method. By this way, all the Zernike terms of the flat are absolutely calibrated and the test accuracy of the interferometer is greatly improved.