An improved two-step phase-shifting algorithm based on the Gram-Schmidt orthonormalization is proposed. After removing the direct current of the interferograms by a Gaussian high- pass filter and orthonormalizing the intensity of the two-frame filtered interferograms by Gram-Schmidt process, the phase shift can be extracted by an arcsine function according to the relationship between two orthogonal vector norms. In the two-step phase-shifting interferometry, as long as the phase shift value between two-frame interferograms is determined, the measured phase can be retrieved by an arctangent function. This method can not only extract the accurate phase shift value, but also retrieve the phase rapidly. By means of computer simulation and experiment, and comparing the performance of this method with several two-step phase-shifting algorithms, the result proves the effectiveness and the advantage of the proposed method in the phase measurement.