The quality of reconstruction in phase-shifting digital holography was determined by the accuracy of phase shift. An approach to obtain accurate phase shift in two-step digital holography was proposed. This approach employed an ordinary open-loop controlled piezoelectric actuator, and a serial of phase shift holograms corrisponding to different phase shift angles would be obtained by means of changing the volgate exerted on the piezoelectric actuator continuously. Then the holograms could be used to reconstruct the object intensity on the CCD plane. The object intensity on the CCD plane that already recorded by CCD would be used as a criterion to evalutae the quality of the reconstuction. The correlation coefficients between the criterion and the same intensity reconstructed with different phase shift holograms were obtained. The correlation coefficient reached its maximum when the phase shift hologram is with the accurate phase shift angle. Simulation was carried out and thereafter the approach was employed in experiment. The quality of the reconstructed field by using the accurate value and the nominal value were compared. The feasibility of this method has been verified by both computer simulations and experiments.