An electrochemical impedance spectroscopy (EIS) sensor is designed based on the principle that the lubricating oil performance will change after the lubricating system enters the water, and an on-line monitoring system for the sealing state of the finishing mill roll box is established By monitoring the change trend of water content in lubricating oil in real time and prompting the maintenance personnel to change the double lip seal of the roll box in time, the problem that the finishing mill can't judge the failure of the double lip seal of the roll box in time is solved by warning and prompting the maintenance personnel to change the double lip seal of the roll box in time.