In sensors based on linear electro-optic effect, half-wave voltage of the electro-optic crystal is one of the key parameters to influence the input and output of sensors. With analysis of the principle of electro-optic modulation, lithium niobate crystal is chosen to be a part of the half-wave voltage measurement system. Measurement of half-wave voltage is finished, theoretical calculation is done, and comparison of the calculation results and the experimental results is completed. Factors influencing the half-wave voltage are analyzed, problems that should be taken care of in engineering application are mentioned.