Extrusion tapping is a novel and important processing method of internal thread, carrying out research on the thread forming process can contribute to a better application of this process in more industrial situation. The principle of extrusion tapping of internal thread is introduced in this paper firstly, and then the simplified finite element model of extrusion tapping and the thread forming simulation is conducted. Next, the thread forming process and metal plastic flow law during extrusion tapping of internal thread are analyzed based on the simulation. Finally, the revised formulas for calculating extrusion deformation area and volume are presented and comparison are made with the referenced formulas, the results show that the revised formulas can express the practical extrusion deformation area and volume more correctly. Furthermore, the similar law between the revised extrusion deformation volume and extrusion force obtained from simulation are proved effectively.