The invention discloses an online monitoring system. The online monitoring system comprises a mechanical lock used for locking an electric meter box, a microprocessor, a key cuts comparing module, a key address comparing module, a communication module, a buzzer, a wakeup circuit, a sensor and an acquiring terminal. The sensor is arranged in a lock cylinder of the mechanical lock, the sensor is connected with the wakeup circuit, and the wakeup circuit is electrically connected with the microprocessor. The key cuts comparing module and the key address comparing module store key cuts data and key address data respectively. The online monitoring system integrates functions of the common mechanical lock and the online monitoring system, onsite alarm and remote recording and alarming functions are combined by means of the key cuts comparing module and the key address comparing module, states of the electric meter boxes can be monitored conveniently by metering personnel, violations such as electricity stealing can be prevented in time, national property is protected and economic loss is reduced.