In the optical voltage sensor based on first order electro-optic effects, the half-voltage of electro-optic crystal is one of the key factors influencing the input-output characteristic of sensors. With relative analysis of the electro-optic modulation mechanism, LiNbO3 crystal is chosen to set up the measurement system of half-voltage, measurement is done on half-voltage. Theoretical calculation of half-voltage is completed, comparison with the actual results is finished, factors influencing the LiNbO3 crystal half-voltage is discussed, and problems need to be taken into consideration in practical engineering application are pointed out.