The invention discloses a transformer partial discharge on-line monitoring method which comprises the following steps of: (1) arranging a high-frequency current transformer and an ultrahigh-frequency transducer on a ground wire of an iron core, an outer shell or a clamping piece of a power transformer, wherein the high-frequency current transformer and the ultrahigh-frequency transducer are respectively used for coupling a signal flowing through the ground wire; (2) inputting output signals of the high-frequency current transformer and the ultrahigh-frequency transducer to a monitoring device, respectively detecting waves, maintaining amplitude and phase information after respectively subjected to digital-to-analogue conversion, and comparing; and (3) forming a monitoring signal in the monitoring device according to the analysis result so as to monitor the partial discharge of the transformer. The high-frequency current transformer and the ultrahigh-frequency transducer are installed on the ground wire of the iron core, the outer shell or the clamping piece so as to monitor discharge, the structure of the power transformer is not changed, thus the installation is convenient; meanwhile, the monitoring sensitivity is high and the anti-electromagnetic interference capability is strong. The invention also discloses a transformer partial discharge on-line monitoring device.