Abstract:This article studies the criteria for determining the Magneto-Hydro-Dynamic (MHD) instability of aluminum electrolytic cells, with a focus on analyzing the key role of the arithmetic mean of the vertical magnetic field Bz (Bz0) in the evaluation. The effectiveness of Bz0 as a instability indicator for MHD has been verified through literature review, theoretical derivation, and empirical research. The study used a 500kA aluminum electrolysis project as a case study, and the results showed that Bz0 has a significant guiding role in the design of the electrolytic cell magnetic field. This article proposes a instability determination model for MHD based on Bz0, and compares the advantages and disadvantages of different instability criteria, providing new ideas and methods for the design and optimization of aluminum electrolytic cells.