To meet the requirement of phase-shifting measurement of the wavefront aberration detecting stage for lithographic projection objective, based on theory of Shearing Interferometry, a phase-shifting device is designed. Bridge amplification mechanism is adopted to obtain a larger amplification ratio and a relatively compact structure. The theory of the bridge amplification mechanism was analyzed and discussed, and the functional relationship and its curve between input displacement and amplification ratio were obtained. To make sure the availability of this structure, nonlinear finite element analysis was proceeded. And the results meet the requirements of phase-shifting device of wavefront aberration detecting stage.