The voltage phase angle calculated by using DFT algorithm in phasor measurement unit(PMU) contains large errors because of the volatility of the actual power grid frequency. For lacking of the other method in real power grid to measure the phase angle difference, it is impossible to verify the error of phase angle difference measured by PMU. The voltage phase angle difference between different measuring points has not been obtained accurately in actual power grid. This paper presents a method to calculate the phase angle difference of voltage at different measuring points using the voltage waveform data uploaded by the fault recorder. The synchronization of the recorded data measured at different substation is realized by using the zero-sequence-current waveform at the time of single-phase earth fault, and the voltage phase angle difference between different measuring points is calculated by the correlation function. The results of theoretical analysis and actual data analysis show that the accurate voltage phase angle difference at different measurement points of power grid is accurately obtained by this method. With the calculation results of this method, it is possible to modify the measurement error of the voltage phase angle difference calculated by PMU. This method is important to analyze the measurement accuracy of voltage phase angle difference between voltage phasors at different nodes of power grid and improve the value of fault recorder data.