According to the degradation in strength and deformation of structure with intermediate state and failure-dependence, an assessment of seismic reliability of bridge engineering structures is brought forward, which assumed there is a three-stage work mode of safety, intermediate and failure. Freely supported beam bridge is reduced to serial parallel systems. Using base shear of piers and displacement of girders as calculation indexes, models of three-levels of reliability vector are put forward to calculate the reliability vector of piers, girders and bridges respectively. A practical method for dealing with failure-dependence of anti-seismic engineering systems is put forward by considering dependence parameter. The reliability of RC freely supported beam bridge can be effectively predict by the proposed approach. It provide theoretical basis for the assessment of seismic performance and offer accurate datum that are needed for the reinforcement of bridge, so the potential disaster of lifeline engineering can be prevented or decreased.