Shortstress line rolling mill is widely used in bar production line because of its short stress line, big stiffness and high products precision. There were several malfunctions such as the press mechanism was easy to stuck and roller axial movement was very large.The reasonable gap between the press ring and the rod was solved by analyzing the gaps between the main bearing parts of the rolling mill and the deformation during the rolling process.The gap formed by the dimension chain could not be eliminated because it involved more links on the direction of roller axial positioning. The structure of the mill was optimized by adding a positioning slot on the center frame and adding a positioning guide device on the outside of the roll box.The simulation of the rolling mill stiffness on old and new structure was carried out. The results show that the improved structure can reduce the axial displacement of the mill and enhance the stiffness effectively at the same time.