The invention discloses a voltage transformer opening triangular loop broken line monitoring device and an application method. The monitoring device comprises a fully-closed cabinet and installation fixing lugs and is characterized in that a display screen is disposed on the front panel of the fully-closed cabinet; an operation keyboard is disposed under the display screen; signal indication lamps are disposed to the right of the display screen; a PCB is disposed in the fully-closed cabinet; the secondary loop voltage terminal strip of a 10kV bus is disposed on the upper part of the rear side of the fully-closed cabinet; a 485 communication terminal strip is disposed on the lower part of the rear side of the fully-closed cabinet; and an Ethernet interface is disposed on the right part of the rear wall of the fully-closed cabinet. The monitoring device and the application method may timely give alarm information and notify a scheduling main station when a broken line is generated in the opening triangular secondary loop in order that a maintainer treats and removes hidden dangers in equipment operation, guarantee the reliability and the sensitivity of monitoring results, and guarantee defect-free operation of the opening triangular loop.