Abstract: To study half-wave voltage measurement in the electro-optic modulation experiment, the main relationships including double-frequency characteristics and linear characteristics between output signal and modulation signal are analyzed based on the principle of crystal electro-optic modulation. The influence of the amplitude of modulating signal on the frequency doubling signal is investigated by computer simulation. The analysis shows that the frequency doubling signal waveform is distorted under the impact of high order harmonic wave, which is not in favor of crystal half-wave voltage measurement. Symmetry of Lissajou figure is used to determine the position of the frequency doubling signal and overcome the disturbance of modulating amplitude waveform aberration. Compared to other methods, this method is easy to use and can measure half-wave voltage accurately.