For necking extrusion forming process of seamless steel tube with large diameter, variation of process parameters and method to improve the deformation extent of necking were studied. The relationship between process parameters such as semi-cone angle, heating temperature, forming velocity, friction coefficient and limited necking coefficient was analyzed and optimized by finite element simulation software Deform-3D. Based on theoretical study, the method of localized heating with a temperature gradient was put forwarded to effectively increase the deformation extent of necking, and the values of temperature gradient were optimized. The analysis results show that limited necking coefficient mmin is relative to the semi cone angle α and the friction condition, and at the same time the best semi-cone angle is about 23°. The localized heating with a temperature gradient can increase the necking extrusion remarkably. The experimental results show that scaling ratio (size，1∶0.6) experimental result of necking coefficient m under uniform temperature field is coincident with the simulation result. In the necking extrusion forming experiment (size，1∶1), the necking coefficient m reaches about 0.67 and the necking deformation extent increases about by 15% when the temperature gradient value is 1.67.