Preloading the high strength bolt using hydraulic extension method is a fast and efficient method for bolt installation. Hydraulic bolt tension device can guarantee the initial tension of bolt well. However, if the relation of initial tension and the final remaining preload of bolt cannot be determined, the preload on the bolts still cannot be guaranteed. This paper uses theoretical analysis method to research the relation between the eventual preload and the initial tension of high strength bolts on Heavy-duty Gantry beams. Then we proposes a set of calculation method of bolt installation preload and the initial tension and provides theoretical basis for the installation of heavy-duty segmented beam.