To realize high-speed, accurate, and sensitive measurement of electro-optic (EO) coefficients, a novel measurement method using photoelastic modulation is proposed. A simple polarimetry is constructed based on the photoelastic modulator, and the phase difference induced by the EO sample is loaded in the modulation signals. Employing the digital phase-locked technology, the DC, first, and second harmonic terms are extracted, respectively, and then the EO coefficients are demodulated. The experimental system is set up to analyze the principle of this scheme in detail. A Y-cut LiNbO3 crystal plate is selected as the sample, and the EO coefficient γ22 is studied under different applied DC voltages. The results demonstrate that the measurement accuracy and sensitivity of the system are 0.22% and 1.5×10-14 m/V, respectively, when the sampling time is set as 198.9 ms. The measuring rate can be further enhanced, if the appropriate reduction of the measurement sensitivity is allowed.