In this study, an iterative phase-shifting algorithm based on the least-squares principle was de. veloped to achieve multi-surface interferometry. The accuracy of interferometry was further improved by incorporating the secondary reflection signal into the least-squares solution equation. First, according to the least-squares principle, the theoretical value was correlated with the actual interference information to derive the least-squares equation. This derived equation was divided into two parts: the iterative calcula. tion of the initial phase and the iteration of the phase-shifting value between two successive frames. From the iterative results of 21 interferograms, an accurate initial phase distribution was obtained. The simula. tion results show that the solution accuracy without considering the secondary reflection signal is 10 nm, and the accuracy improved to 0. 3 nm when the signal is considered in the calculation. In addition, the experimental results of the measured data indicated that the developed algorithm can be used to reconstruct a more accurate phase distribution, and no clutter interference or residual harmonic signal is observed in the results. Therefore, the developed algorithm can be used to achieve high-precision multi-surface measure. ment, which has research value and practical significance.