The time difference of a UHF partial discharge（PD）signal for PD location is always calculated from the time domain waveforms

however

when the signal to noise ratio（SNR）is low or the initial pulse of the PD single is small

it’s difficult to obtain the arrive time of the initial pulse.This paper proposes a new algorithm for achieving the time difference

which is combined with the characteristics of EM-wave modes in GIS and changes the PD signals from time domain to frequency domain by using fast Fourier transformation.A simulation model is built by the finite-difference time-domain（FDTD）method for obtaining the theoretical time difference of PD signals and analyzing the anti-interference ability of the new algorithm.On the experimental GIS model both internal and external antennas are used to obtain the time difference of real PD singles so as to analyze the internal factors of impacting PD location. Simulation and experiment indicate that the proposed algorithm can obtain the time difference of PD signals in GIS with strong anti-interference ability.