The form changing of a billet has been studied and a deformation ratio calculation of metal depending on a mandrel tip position in the deformation zone during the helical piercing process has been carried out. To solve the problem the finite-element simulation method with use of the program complex DEFORM-3D v10.0 was applied. It was found out that the mandrel position in the deformation zone had a considerable effect on the metal flow character in the deformation zone. In addition the mandrel position influences on increase in a length and decrease in a number of helix leads as well as a growth of the deformation ratio in a close-contact area of the billet.