The value of the half-wave voltage of the phase modulator must be measured with high accuracy to reduce the quantum bit error rate (QBER) in a quantum key distribution system based the phase-coding scheme. A new method to measure the half-wave voltage of the phase modulator in an accuracy of 2 mV by adjusting the quantum bit error rate (QBER) of the deterministic quantum key distribution is proposed experimentally to increase the accuracy of the half-wave voltage and reduce the error rate of the quantum key distribution system. The experimental results show that this method can be used to acquire efficiently the half-wave voltage of the phase modulator in a high accuracy which can make the error rate from adding the inaccurate voltage to the phase modulator decrease to the greatest extent.