The reasons for the phase shifting errors in a phase shifting interferometric measurement were analyzed, and a method to correct the phase shifting errors was researched based on the least square method. The calculation and correction for the relative phase step amount between arbitrary two interferograms were implemented. With the combination of optical, mechanical, electrical and calculating subsystems, more than three step interferometric images with arbitrary intervals were achieved. Then, the iterative least-square algorithm was used to calculate the relative phase step amount between arbitrary two interferograms. The phase shift error was feedbacked to the hardware system and the phase step interval was corrected in self-adaption until the phase step interval was equal to the characteristic value of the phase shift. A phase shift error correction system based on iterative least-square algorithm was constructed to achieve phase step error self-correction on a phase shifting interferometry. The real measurement of optical surface was conducted to verify the correction of algorithm and the feasibility of phase step error self-correction system. The results show that the relative phase step error is less than 5%, and the root mean square error(RMS) for measurement repeatability of the optical surface is less than λ/1 000 with the help of the proposed phase step self-correction process.