The invention relates to an online monitoring device for the partial discharge of GIS, and the device comprises a supersonic data collection unit, an ultrahigh frequency data collection unit, a data processor, and a main unit. The invention also discloses a positioning method for the partial discharge of the GIS. Through the comparison of the time domain waveforms of an ultrahigh frequency signaland a supersonic signal of the partial discharge of the GIS, the device determines the position of a partial discharge power supply of the GIS according to the time domain waveform signal time difference collected by an electromagnetic wave sensor or a piezoelectric sensor through a time difference positioning method or a sound-electricity combined time difference positioning method. The method, which combines the ultrahigh frequency live-line detection technology with the supersonic live-line detection technology and is used for locating the partial discharge power supply of the GIS, is highin positioning speed, and is accurate in positioning.