Abstract: Analyzed in the paper is the forming mechanism of the cavity occurs in the center of a piercing shell. Based on the principle of slip line field，the mechanical factors leading to the formation of a cavity is elaborated.To avoid the formation of a cavity，the idea of improving the stress condition at the center area by adding a transversal compression stress is put forward，and so is the method of reducing the distance between the guide shoes.The application results indicate that when the distance between the guide shoes is adjusted from 151 mm to 149 mm，it is able to prevent the insufficient plug lead from causing a cavity，and to improve the homogeneity of pipe body wall thickness.