For phase-shifting interferometry, the inter-pixel algorithm with ellipse fit is a kind of effective method to get the phase shift steps. The key of improving the precision is to search two groups of pixels, whose phase difference is π/2, from the interferogram. Therefore，The two interferograms with phase difference π/2 are chosen by calculating the zero and peak positions for local gray mean values of all interferograms. Using these two interferograms, a method is presented to find two groups of pixels of phase difference π/2 with maximum number for the whole image based on four gray comparison methods. The experiment shows that the proposed method has higher accuracy and universality than the traditional method.