In order to realize the high-precision surface measurement of large-diameter elliptical optical flat mirrors and improve the image quality of large-aperture telescope systems, the absolute measurement algorithm for flat elliptical mirrors is studied in this paper. Firstly, the orthogonal polynomials fitting of an elliptical optical flat mirror is studied. Then, the absolute testing algorithm is studied theoretically. The orthogonal absolute testing algorithm can effectively separate the surface error of the reference mirror from the mirror to be measured, which can realize the high-precision surface reconstruction of the elliptical flat mirror to be measured. To verify the actual testing accuracy of the above method, we carried out an absolute testing simulation and experiment on a 250 mm×300 mm mirror. In the simulation, the possibility that the reference surface error is high was considered. In the experiment, a 250 mm×300 mm elliptical testing area was selected in the Zygo300 mm standard flat surface. The above-mentioned elliptical area was tested by the 150 mm Zygo interferometer, and the surface reconstruction was realized based on the above-mentioned orthogonal absolute testing algorithm. The experimental results show that the surface error separation between the reference mirror and the elliptical mirror can be achieved by using the method described in this paper, and the residual RMS (Root-Mean Square) value of the absolute testing result is 0.29 nm, which proves the feasibility and accuracy of the method described in this paper. The high-precision surface reconstruction of the elliptical flat mirror can be achieved using the above method.