Abstract: Briefed in the paper are causes of "fishtail" defect on pierced shells,and accordingly put forward are improvement measures focusing on aspects such as adjustment of the piercing mill center line and other process parameters,checking of tools and dies,and preparation of billets etc.The analysis shows that by controlling precision of the center line,optimizing process parameters and billet heating temperatures,changing severely worn tools and dies,controlling billet cutting quality,and adopting billet end centering method,the probability of "fishtail" defect occuring at a shell end will be significantly decreased,the life of the mandrel increased,the roller changing time shortened,and product quality improved.