Characteristics of phase modulation of a nematic liquid crystal is studiedo Based on the study, a new phase-shifting shearing ESPI system which use parallel-aligned liquid crystal as a phase modulator is set up. This method increases the mearsuring accuracy of shearing ESPI.The method calculate phase simply by using correlation fringe and modulate phase simply by using correlation fringe and modulate phase very easly. From the experiments to measure the deformation of an object, we acquire a satisfactory result.