Abstract: Least square method is used to fit power and astigmatism of rectangular pupil wavefront data, which avoids data interpolation of the blank area in circular domain. In the experiments, the maximal difference of power and astigmatism of circular domain data between commercial software and our method is less than 0.005 wavelength, which proves the proposed methods feasibility. For rectangular pupil wavefront data, the differences of astigmatism increase as the rectangle elements length-width ratio increases. Therefore the proposed method is more applicable for rectangular pupil wavefront calculation.