The design of a new pre-tensional structure(FROMO® preload nut) applied in the large size of screw thread was introduced, the preload bolt is only beared with the pure tensile stress. The pre-tightening force for the preload bolt and pre-tightening torque for the jack bolts were calculated. The practice elongation value of the preload bolt was measured, and compared with its theoretical elongation value. The result shows that: the new pre-tensional structure has adequate strength, it can control the pre-tightening force accurately with manual acting spanner, it increases efficiency and ensures the safety of operating , and which has promised the good performance in the application of the projects.