It's needed to locate the PD Source for identifying the Insulation defect which induces PD to happen and eventually causes Insulation breakdown in gas insulated switchgear (GIS). A simple and obvious way of PD localization is the time of flight method which is widely used as a part of UHF method. But in application, the time of flight method can hardly be applied effectively and will induce errors if the initial pulse of the PD signal is small and blurry or its signal-to-noise ratio (SNR) is too low. For improving the accuracy of PD localization, the characteristics of electromagnetic wave (EM-wave) propagation route in GIS were investigated in this paper. The simulation and experiment results show that the EM-wave signals induced by PD in GIS are related with EM-wave propagation routes, which were decided by the location of PD source and detector's position. As a result, a PD localization method in time domain based on EM-wave propagation route was proposed. The experiment results show that the proposed method can locate the PD well and effectively in the case that the initial Pulse of PD signals is very small and blurry when PD source locates near the outer conductor of GIS.