Aluminum production is a high energy consumption process so that maintaining fundamental compositions in balance and optimal conditions are essential. The molten electrolyte and melted aluminum are primary materials and their boundary needs to be monitored from time to time. An automatic measurement technic is presented in the paper to substitute for the traditional manual measurement work that is dull, poor efficiency and dangerous for operators. The boundary forming mechanism is analyzed, the vertical profile of electric potential is simulated, an automatic in-strument is developed to sense the potential distribution, and a strategy is provided to identify the boundary according to the potential curves. Finally, some practical results are compared with manual measurements, which shows good consistency.