Measuring the half-wave voltage with high accuracy and a simple setup is an important issue for the applications of the electro-optical phase modulator. A new method was proposed which was based on the principal of beam steering in the laser phased array. The relationship between the offset of central beam in far field and the half-wave voltage was formulated by theoretical analysis. The setup of the 1×2 all-fiber laser phased array system was built and the half-wave voltage of the LiNbO3 waveguide phase modulator was measured. Several far-field intensity distributions with different loading voltages were recorded and the measuring error was decreased by taking average of the multiple results. What’s more, the curve of phase modulation characteristic was obtained according to the multiple far-field intensity distributions. The experimental results show that the proposed method is simple with high accuracy and has important applications in engineering. Meanwhile, the phase modulation linearity can be analyzed.