It is quite common in highway engineering that building a bridge across a debris flow gully to prevent roadbed from damage by strike of debris flows. As bridges are designed with the purpose of protection to their piers against debris flows, it is crucially important for engineers in determining the magnitude of the Impact Force Exerted by Boulders Transported (IFEBT) in rush torrents. In view of the theory of energy conservation, a formula was introduced in this paper to calculate the IFEBT with appreciable improvement as compared to the common equations, in which either cantilever structures or simply supported structures, only the two types of structures were taken into account in modelling. The Thornton elastoplastic contact criterion was included in the formula in consideration of buffer effect of two-phase debris flow on bridge piers and dynamic responses of bridge upper-structure. Comparisons on calculation accuracy were elaborately made between our improved formula and the used one by a case study of Den Jigou Bridge. It was found that according to our proposed method the values of IFEBT obtained in circumstances of varied velocity and sizes of boulders were lower than the ones gained by the used methods. Furthermore, providing the depth of debris flow body in the two-phase condition were up to 2.4cm, there were a considerable decrease of 21% in the value of IFEBT. In the meantime, a decrease of 1.4% in the IFEBT value was attained in consideration of the action of inertia force of the upper-structure of bridges. In addition, it is feasible to dissipate impact energy of IFEBT when low elasticity modulus and high decrement material were used in practical engineering.