The invention discloses a method of locating a discharge source for a GIS ultrahigh frequency partial discharge online monitoring device. At present, the ultrahigh frequency partial discharge on-line monitoring device cannot use the time difference between the signals to achieve the precise location of the partial discharge source because the processing sampling rate of the detected peak signal of the ultrahigh frequency partial discharge on-line monitoring device is low. The method uses the GIS ultrahigh frequency partial discharge on-line monitoring device to collect the ultrahigh frequency signal of the GIS equipment, uses the Gaussian smoothing filtering method to extract the envelope of each ultrahigh frequency signal, calculates the time domain cumulative energy function of each ultrahigh frequency envelope signal and performs the normalized processing, calculates the inflection point of the normalized time domain cumulative energy function curve as the signal energy sudden time, i.e., the signal start time, calculates the difference value between the start times of the respective signals, and uses the discharge source positioning formula to calculate the discharge source position. The method can directly use the low sampling signal of the GIS ultrahigh frequency partial discharge on-line monitoring device to perform time difference positioning.