Abstract: The tank top to tank shell of dome roof tank is a thin-connection structure and is easy to crack during vapor purging. The analysis on the design regulation of dome roof tank and its stress conditions is made. The results show that the vapor purging operation time is a key to prevent the tank top from cracking. The measures to control the vapor purging time and flow rate of vapour, enlarge the oil-gas space and increase the number of air hole as well as venting area are given, which can effectively prevent the tank top from cracking.