A lightning rod on a coastal substation cracked, so the related equipment is forced to be shut down. Optical microscope, spectrometer, SEM, EDS and ion chromatograph were adopted to analyze the corrosion mechanism. Results show that the lightning rod is corroded severely. The material of this lightning rod is 20# carbon steel. The corroded part and the uncorroded part are divided into two layers. Nodulizing of pearlites is evident in the microstructure of corroded area. Both of EDS and ion chromatograph detect high concentration of chloride ion and sulfate ion from the corrosion products. In this sense, corrosion is probably induced by coastal-industrial atmosphere and rainwater pollution. Besides, improper structure of the lightning rod is also a factor to cause the corrosion.