The mechanism of corrosion on the surface of the naval gun barrel was studied in view of the phenomenon that the chromium plating layer on the surface of the gun tube was peeled off, scattered in irregular and uneven depth. Through the macroscopic and microscopic corrosion morphology of the corroded surface of the sample and the statistical analysis of the distribution rules of the corroded pit, it is concluded that the cracking and peeling of the chrome-plated layer is the fundamental reason for the corrosion of the barrel. The experimental results show that the corrosion medium reaches the interface of matrix and forms corrosion source, in which chloride ion plays a pioneer role. The fracture of chromium layer during bubbling and expanding of corrosion pit is a fixed-branch bending fracture mode. This study can provide reference for the surface corrosion prevention and corrosion repair methods of naval gun.