Firstly, the generalized Hamilton’s principle was used to establish a model for bridge structure, which was discretized and transferred into Hamiltonian system to get the canonical equations. Secondly, the idea of PIM(precise integration method) was introduced into the Symplectic geometric algorithm to establish a Symplectic PIM(SPIM).This new algorithm was then applied to deal with moving force identification for bridge structure, from which the vibration equation was sine/cosine transformed to get the SPIM format. Compared with RungeKutta method and the PIM format for load identification, two numerical results display the longterm stability properties and the high precision of the SPIM, whose results could not be affected by the timestep .As a result, the timestep can be enlarged to speed up the computation.