The operating principle of photoelasitc modulator with the length stretching vibration has been investigated and the vibration analysis of the photoelastic modulator has been carried out. The anisotropic piezoelectric quartz vibrates at the driving voltage. When the frequency of the driven square wave signal matches the intrinsic frequency of the two crystals, the fused quartz which is flexibly connected with the silicon rubber of the piezoelectric quartz will resonate. The longitudinal standing wave with same frequency propagates in the two crystals, while the photoelastic modulator as a whole vibrates with onedimensional length stretching. The vibration of photoelastic modulator can be equivalent to a damped massspring system and then the expression of the amplitude can be derived. The vibration displacement is proportional to the Q values of the driven voltage and the photoelastic modulator. Finally, the vibration characteristic of the homemade photoelastic modulator has been measured through the finite element simulation and the vibration measurement experiment to verify the validity of the theoretical derivation.