When the quartz(SiO2) is used as the piezoelectric actuator of a photoelastic modulator(PEM), it has shortcomings of smaller electromechanical coupling coefficient, high driving voltage required, and serious frequency drifting with a temperature. Therefore, this paper designs an optimized PEM. In consideration of the special crystal structure of lithium niobate (LiNbO3), the feasibility of LiNbO3 crystal to be a piezoelectric actuator was verified and its cut shape was set as zyw/35°cut. Based on the finite element analysis software COMSOL4.3a simulation, the chip size and the resonant frequency were determined. Then, a LiNbO3 piezoelectric actuator was designed. The piezoelectric performance of the piezoelectric actuator was tested and compared with SiO2 piezoelectric actuator. Finally the LiNbO3 piezoelectric actuator was combined with a ZnSe crystal to construct a PEM, and it was performed a photoelastic experiment by a 671 nm laser. The experiment for achieving the same displacement show that the drive voltage for the SiO2 based piezoelectric actuator is 100 times that of LiNbO3 based piezoelectric actuator, and the unity and stability of lateral length stretching vibration mode for the latter is better than that for the former. Moreover, when the drive voltage is 3.76 V, the optical path difference of 671 nm laser will be modulated to 3.7 μm through the LiNbO3 piezoelectric actuator based PEM. These results demonstrate that proposed LiNbO3 piezoelectric actuator based PEM is easy to drive and control and the modulation performance is better than that of previous PEMs.