The work aims to study the leakage of SF6 from GIS cylinder 8 mm-thick aluminum tube in air chamber of a main transformer in Zhejiang offshore, which has only operated for 3 a and then determine the cause of the accident. Through the technical means of optical spectrum, metallography, SEM, EDS, and XRD etc, the possible causes of the leakage of SF6 and pitting in the aluminum tube were discussed from the aspects of material, structure, environment, etc. Corrosiveness of simulated environment was validated by electrochemical testing. Then, the effect of fireproofing mud on accelerated pitting perforation of 5052 aluminum tube was revealed. The substrate material of corrosion sample was 5052 and the chlorine content of fireproofing mud in contact with the etched sample was up to 24.5wt%. According to the GB/T 12007.3—1989, the inorganic chlorine was about 3.3wt%. The dense microcracks caused by corrosion in turn promoted the transmission of corrosive medium. Furthermore, area where the GIS was located lied on the coast. Annual Cl- content of rainfall was up to 1.8 mg/L in the region in 2012. Rainfall was obviously acidic and had a pH of 4.64 at the same year. Both pH and Cl- content were beneficial to autocatalystic reaction of aluminum. The high chlorine content of fireproofing mud is the main reason for the rapid pitting and perforation of the aluminum tube of the GIS cylinder, and the microcrack structure, acid rain and the coastal atmospheric environment have a promoting effect.