Phase-shifting interferometers with three piezoelectric transducers (PZTs) on their phase-shifting adapter (PSA) are widely used in precision measurement. The guarantee of the PSA to stretch as a flat is very important to the accuracy of the measurement. In this paper, we propose a systematic self-calibration method to calibrate the different displacements of the PZTs so that the PSA can be moved evenly as a flat. The proposed method uses only the interference patterns to calibrate the three PZTs and information about the exact position of the PZTs and the instrument parameters of the interferometer are not required. Experimental results show that, after calibration by the proposed method, the deviation of the measured phase shift error could be reduced about 43.1 times from 3.492 to 0.081°.