Abstract: During the hot rotary piercing process of the large-sized seamless steel tube,spiral defect or spiral-fin defect occurs on the external surface of the pipe due to the fact that the tool guide plate in the deformation area gets partially softened and glued or bitten. The causes for the said spiral defects of the pipe are investigated and then elaborated means of analyzing all the affecting factor such as the metal rotary-rolling deformatian principle,the process parameters and the process guide-plate die,etc. Then improvement measures are put forward,involving the shape design and chemical composition of the die guide-plate,etc. as well as actions to prevent the above mentioned spiral defects.