Abstract: In order to find out the causes for local cracking in ZL305 aluminum alloy casting after many years of service, optical microscope, scanning electron microscope and chemical analysis were used to analyze the cracking position. It is found out that the cracking of the casting belongs to stress corrosion cracking. The reason is that the magnesium content in the alloy exceeds the upper limit of the standard, and β phase precipitates gradually at the grain boundary and takes on chain shape during the service, which reduces the corrosion resistance of the grain boundary. In addition, the alloy bears large stress in the marine environment for a long time, resulting in stress corrosion and cracking.