Abstract: Since the measurement accuracy of a phase shifting interferometer is deeply affected by nonlinear phase shifts, a novel phase shifter calibration method based on iterative least-square-fit was proposed to cope with it. Several interferograms could be taken after applying voltages to the phase shifter, and the phase shifts between interferograms could be calculated by iterative inter-and intra-least-square-fit, so the relation curve between the applied voltages and phase shifts was obtained, then the curve was fitted and the voltages were finely adjusted and finally the phase shifter was calibrated accurately. The proposed calibration method is verified on a modified interferometer, and the measurement result is compared with that of a Zygo interferometer with the same component, the RMS of the difference between two results is 1.726 nm. It can be seen that the proposed calibration method can help to get high accuracy measurement result with a nonlinear phase shifter.