Abstract: Described here in the essay is the design method for developing the large-sized extra-heavy-wall,hot-rolled seamless steel tube with the cone type piercer(CTP)slight-expansion/piercing technique.Detailed are the method for determining the rolling angle and the deformation section length of the CTP,and the method for designing the piercing plug with both the CTP pass calculation formula and the 3-D model-setting process.Finally thanks to the proper CTP pass design and the CTP slight-expansion technique,the extra-heavy-wall seamless steel tube with D/S≤5 is actually produced.The successful development of the said CTP slight-expansion/piercing process lays a foundation for further expanding the application market of the seamless steel tube.