The partial discharge （PD） signals produced by a fixed protrution were measured by two PD measuring methods in a simulating system of Gas Insulated Switchgear （GIS）. A way of noise rejection by a powerful tool. i. e. the wavelet transform was used in the system. Because of the local focus characteristics of the wavelet transform

the main characteristics of the PD signals could be abstracted under serious noise background. This is an application of an algorithm that removes noises from signals by analyzing the evolution of the wavelet transform maximums across scales. It is clear from the analysis that the signal processing on the basis of wavelet transform is really superior to the traditional method of spectrum analysis of FFT.