The invention belongs to the field of monitoring of looseness of a cantilever screw of an overhead line system of an electrified railway, and discloses a monitoring system for the looseness of the cantilever screw of the overhead line system of the electrified railway. The monitoring system adopts a method for obtaining a vibration spectrum of a cantilever of the overhead line system of the electrified railway through a vibration sensor, a microprocessor and a time domain frequency-domain algorithm. The monitoring system comprises the vibration sensor, the microprocessor, an outer part communication interface, a power supply and an outer shell. The system converts vibration of the cantilever into electric signals through the vibration sensor to be transmitted to the microprocessor, the microprocessor calculates the vibration spectrum of the cantilever through the time domain frequency-domain algorithm, the calculated vibration spectrum of the cantilever is compared with a saved vibration spectrum under a normal state of the cantilever, and when error occurs, looseness of the cantilever screw can be considered. Monitoring of the looseness of the cantilever screw of the overhead linesystem of the electrified railway is realized.