In the piercing process of the large-sized seamless steel tube, the rollers and plug affected on the billet with extrusion and friction. It made that the stress and strain state of metal is very complicated. In the complex state, the casting shrinkage and void produced by molten steel solidification became the crack defect in the middle of tube wall. The simulation of the piercing process of large sized seamless steel tube was performed by FEM in this paper. The material is stainless steel TP321. The distribution of stress was analyzed. It was obtained that the circumferential shear stress is the necessary condition of produced the layered defects in the tube wall. The paper studied the state of strain in the tube. The distribution of all kinds of strain was gotten. That could provide the theory guide for the elimination of the crack defect in the large-sized seamless steel tube.