By analyzing the electro-optical effect of crystal and varied refractive index with electric field, a mathematical model of electrooptical modulation is established and a new half-wave voltage measurement method is proposed. Based on the mathematical model between the amplitude of voltage applied at both ends of the crystal and the output waveform of system, the half-wave voltage of crystal is deduced by determining the output distortion point. The characteristics of output waveform near the half-wave voltage and the relationship among the light source, power supply, time resolution and the system error are analyzed. Accordingly, two optimization schemes based on time resolution optimization and symmetric optimization are proposed, respectively. Specifically, the time resolution optimization amplifies individual waveform details to reduce data dispersion and error. While the symmetric optimization is to use the symmetry characteristics of corresponding waveform extreme points near the half-wave voltage and take the average or median value through multiple samples to reduce the error. The results show that the critical value method is a simple and effective method for measuring half-wave voltage, whose optimization scheme is efficient in reducing data fluctuation and error.