Abstract: In the ultrasonic thickness mesurment process,the thickness of a Q345R steel pressure vessel in many places was abnormal.The internal defects of abnormal thickness part of pressure vessel were analyzed through chemical composition analysis,mechanical performance test and microstructure observation.The results showed that the manganese content in abnormal thickness of pressure vessel was too high,resulting in a large number of MnS inclusions and abnormal supercooled structures such as bainite and martensite in center of thickness.During the manufacturing process,due to external force or temperature stress during thermal processing,stress concentration occurred at the location of inclusions in supercooled structure and the interface between supercooled structure and ferrite,resulting in the formation of microcracks.