The numerical simulation was implemented using FEM software DEFORM-2D to study the influence on deforming quality caused by sheath wall thickness and sheath bottom thickness. The results show that when the deformation degree is the same, the density distribution uniformity of the billet becomes better along with the increase of sheath wall thickness, but poor with the increase of sheath bottom thickness. Under the conditions of different sheath wall thicknesses, the billet density distribution uniformity first gets worse and then gets improved with the increase of deformation degree, and the uniformity becomes the best when the deformation degree is 80%. Under the conditions of different sheath bottom thicknesses, when the deformation degree is less than 62%, the density distribution uniformity of the billet gets worse first, and then gets improved; when the deformation degree is over 62%, it becomes better as sheath wall thickness increases.