In order to solve the location problem of hot backward extrusion billet，the external positioning clamp mode was adopted which took the billet whose diameter is less than the inside diameter of die into the center of die, so that the punch, blank, center of die stayed in the same axis during hot backward extrusion process to ensure the stress uniformity of metal. Using Deform-3D software, aiming at the simulation of blank whose diameter is smaller than the inside diameter of die during backward extrusion process，the forming effect and distribution of stress in the process of metal forming were analyzed and verified by process test. The result shows that the simulation and experimental results are basically consistent. In hot backward extrusion process, the forming effect is good, and the metal equivalent stress distribution is uniform. Through process test, the method has the advantages of simple operations, less  oxide coating, high precision, and its applicable to all kinds of billet locations whose diameter is less than the inside diameter of die, which meets production requirements.