The present invention discloses a method for on-line monitoring dielectric loss of capacitive power equipment and device. It adopts a high-accuracy microcurrent sensor cover-connected on the earthing wire of equipment, and a resistance divider is mounted on the low-voltage output of voltage transformer of transofrmer station, and a computer can be used for making analytical processing and result display so as to implement on-line mointoring of leakage current signal of power equipment. Said device is formed from high-accuracy microcurrent sensor, resistance divide, signal cable, signal modulator, data collecting card, digital I/O card and computer. Said device utilizes ORACLE to implement data management, and has the functions of historical inquiry, report printing, threshold alarm and remote inquiry, etc.