Abstract: As for wall thickness unevenness of the shell produced with the semi-floating mandrel mill,there could be such different types as batch wall thickness unevenness,shall longitudinal wall thickness unevenness and transversal wall thickness unevenness.The causes for shell wall thickness unevenness are identified.Transversal wall thickness unevenness is due to the original wall thickness unevenness of the pierced hollow bloom,some change of the mandrel mill roll groove and deviation of the rolling centerline,while longitudinal wall thickness unevenness and batch wall thickness unevenness are due to geometric inconsistency of the mandrel.Countermeasures are proposed,accordingly.