Abstract: The macro cell corrosion of steel structure at the beach of Chengdao area was studied by in-situ test and laboratory tests. The influence of many factors, e. g. environmental oxygen concentration difference, temperature difference, area ratio of anode and cathode, the stirring rate of sea water and so on, on the EMF and currents of macro cells was analyzed. The factors that effected the cell's polarity and the mechanism of polar reversal of the macro cell were also discussed. In addition, the formulae for calculating the accelerating multiple of anode corrosion caused by the macro cell were derived, and the protection measures of macro cell corrosion in practice were mentioned in summary.