In order to realize the measurement with high speed, high accuracy and high sensitivity of crystal electro-optic coefficients, a novel electro-optic coefficient measurement method based on photo-elastic modulation is proposed. By means of low-pass filtering and digital phase-locked technologies, the data of the DC term and the first harmonic term of the modulated signals are extracted, respectively, then the crystal electro-optic coefficient is demodulated. The principle of this new scheme is analyzed in detail, the experimental system is set up and the electro-optic coefficient of lithium niobate (LiNbO3) crystal sample is measured. The experimental results show that the measurement accuracy is 2.3%, the sensitivity is 1.7×10－14 m/V, and the measurement speed is 100 ms per data point.