To increase the measurement range of optical voltage sensor, a method for regulating and increasing the half-wave voltage of bismuth germanate crystal by varying the cutting direction was proposed. With the electro-optic wave coupling theory, the dependence of half-wave voltage on crystal cutting direction was investigated. The light propagation direction and the electrical field direction of bismuth germanate crystal were determined by the crystal cutting directions. The results demonstrate that the half-wave voltage is increased to 5 times when crystal is cut along direction[-2-0.5, 2-0.5, 0] and [0.219, 0.219, 0.951]. And the half-wave voltage is increased to 12 times when crystal is cut along direction [0.140, 0.275, 0.951] and [2-0.5, 2-0.5, 0]. The influence of light propagation direction on half-wave voltage was also discussed. When bismuth germanate crystal is cut along the standard direction, the variation of half wave voltage is less than 0.06% with angle error of light propagation in the range of ±0.05π. This method of half-wave voltage regulation is also suitable for other electro-optic crystals.