The invention discloses an online monitoring platform for underground cable channels of urban power grids. The online monitoring platform comprises a monitoring center and a movable monitoring unit connected with the monitoring center, the movable monitoring unit comprises an overhead rail, a movable mechanism and a monitoring terminal, the overhead rail is arranged in a cable trench, the movable mechanism is arranged on the overhead rail, the monitoring terminal is arranged on the movable mechanism and comprises a sensor module, a control module, a video monitoring module and a first communication module, a positioning device, an ultrasonic rat repellent and a deinsectization device are arranged on the movable mechanism, the positioning device is connected with the monitoring center, and the sensor module comprises a temperature sensor, a humidity sensor, an open flame sensor, a combustible gas sensor and a water level sensor. According to the online monitoring platform, temperature, humidity, open flame, combustible gas and waterlogging in the cable trench can be monitored, a certain prediction functions are achieved by the aid of monitored information, and the cable trench can be visually monitored in real time.