The invention discloses a device and a method for monitoring a submerged arc furnace on line in real time. The device comprises a thermocouple array, a main junction box, a data acquisition device and a data processing device. The thermocouple array is buried in the furnace bottom and the furnace wall of the submerged arc furnace according to a point distribution rule, is used for collecting temperature data and is connected with the main junction box, the main junction box converts the data and sends the data to the data collecting device, the data collecting device preprocesses and stores the data, and the data processing device is connected with the data collecting device and analyzes and processes the data. And displaying the real-time state of the furnace bottom and the furnace wall of the submerged arc furnace during smelting. The problem that the temperature and the erosion state of the furnace bottom and the furnace wall in an existing submerged arc furnace cannot be monitored on line in real time is solved. The erosion state, the electrode height and the molten pool morphology can be predicted in real time, safety accidents can be effectively prevented, and the safety of the production process is ensured; manual inspection frequency is reduced, maintenance cost is reduced, and production efficiency is improved.