Abstract: This paper discusses the breakdown voltage of MMO (mixed metal oxide) net anode and oxygen depolarization for the large-scale storage tank. To find out the real CP effectiveness of external CP of storage tank, the Draft Group of 《The Code of Steel Storage Petroleum Tank Corrosion Control Engineering Technology》carries out an experimental analysis on the protection data of MMO net anode and Anodeflex used in large-scale storage tanks located at Zhenhai, Zhejiang Province, Maoming, Guangdong Province and Shanghai, and the results show that there exist a big difference between switch-off potential and switch-on potential when using MMO anode and it is very hard to reach the polarization criterion at -0. 85 V(CSE) as well; while that of Anodeflex is small, which is close relative to the reaction product of anode.