The invention provides a method for monitoring a secondary pressure plate compulsory locking device and the state thereof, and belongs to the field of apparatuses for monitoring the states of put-into-use and out-of-use of secondary pressure plate of an electric power system. The invention involves the secondary pressure plate compulsory locking device, a scheduling main station server, an anti-error host and a computer key. A pressure plate can be compulsorily locked, and completely broken apart, and the states of put-into-use and out-of-use of the pressure plate are effectively recorded. The secondary pressure plate compulsory locking device can be installed while a secondary pressure plate is under an operating state, and on-line monitoring the states of put-into-use and out-of-use of relay protective pressure plate of all stations at the scheduling main station can be realized through telecommunication.