When applying phase-stepping interferometry to measure a surface, distortion will be caused as a result of various error factors especially the phase stepping error from the phase shifter. Even seriously flexuosity can be produced. Aiming at the main error sources to the phase stepping interferometry——the phase stepping error from the phase shifter, theoretical analyses was made for phase stepping algorithm by applying Fourier method. Based on the theory of characteristic polynomial, a new phase stepping algorithm, which is insensitive to disturbance and error, was put forward and improved. Experimental results show that the algorithm has effectively improved the measuring accuracy of the phase stepping interferometry, and made the measured results more approach the actual numerical value.