By employing the new measurement information—rate of phase difference measured by two-element antenna array (interferometer), a passive location systems can find the position of enemy emitters more quickly with higher precision. The accuracy of phase difference is critical to this passive location system. A digital method for measuring it is presented. Spectral analysis on the signals in the measuring system is given, and the signal to noise rate is calculated, then the theoretical accuracy of phase difference is derived. The accuracy is also verified by the simulation results given in the paper.