Abstract: In the conventional ultrasonic thickness measurement of a 37Mn steel seamless gas cylinder, it was found that the local position thickness of the cylinder was abnormal. The abnormal thickness reasons were analyzed by ultrasonic testing, chemical composition analysis, metallographic examination and energy spectrum analysis. The results show that there were delamination defects in the local position of the seamless gas cylinder, resulting in abnormal thickness, and these defects were composed of Al₂O₃ and a small amount of CaO, MgO, TiO₂ and other inclusions.