An iterative method to extract actual phase shift in two-step phase-shifting digital holography is proposed, in which peak signal-to-noise ratio (PSNR) of the reconstructed reference wave is taken as a judging condition. The relationship between PSNR of the reconstructed reference wave and phase shift error is investigated. According to the 1st and 2nd order derivative of PSNR, the iterative direction and whether PSNR undergoes a maximum can be determined. Optical experiment is performed to demonstrate its validity.