It's necessary to accurately locate the partial discharge（PD） source to find the insulation defect when PD happens in the gas insulated switchgear（GIS）. The ultra-high frequency（UHF） based time of flight method is widely used for its merits such as simple principle， easy implementation， and high location accuracy. But if the initial pulse of the PD signal is small and blurry， the time of flight method can hardly be applied effectively but will induce rather big errors. To improve the precision of location， the propagation route characteristics of electromagnetic waves excited by PD in GIS are investigated. It is shown that the electromagnetic wave propagation route is closely related to the site of PD source and the position of the detector. A PD location method based on the characteristics of the electromagnetic wave propagation route is proposed. Experimental results show that the locating method is precise and effective when the PD source is near the shell of GIS and the initial pulse of the PD signal is very small and hardly distinguishable from background noise.