Abstract: If the large-scale flange of pressure vessel is designed as per the Chinese national code GB150,the stress of the flange obtained based on the code is always lower than the actual value.If the closure head of the vessel consists of the large-scale flange and the head cover with the two of them weld together directly,the stress calculation error obtained by GB150 will be even larger.This error is caused by the analysis method of the flange in GB150.Taken the containment vessel of 5 MW Nuclear Heating Reactor as an example,which was designed by Tsinghua University,the shortage of the large-scale flange design based on GB150 is introduced.