To verify the effect of the application of a fluid viscous damper (FVD) and friction pendulum bearing (FPB) in large-span and long-unit bridges with a seismic isolation system, we established a finite element model of a prestressed-concrete continuous-beam bridge. By inputting three artificial seismic waves with a 50-year exceedance probability of 2%, we conducted an isolation study of the bridge with an individual or combined use of an FVD and FPB. With respect to energy dissipation, we determined the combined action mechanism of an FVD and FPB in large-span and long-unit bridges. The results indicate that if only an FVD is used, the FVD will dissipate energy effectively because of its long-period characteristics, but seismic damage to a fixed pier cannot be avoided. Using only an FPB will cause bearing displacement beyond the design specifications under longitudinal (horizontal) strong earthquakes. The isolation mechanism operating under the combined action of FVD and FPB is that the FPB provides a weak connection between the pier and beam, generates relative velocity between them, and thus facilitates the effective dissipation of energy by the FVD. In addition, the FPB serves as an auxiliary energy dissipation device, with the FVD being the main energy dissipation device controlling the beam displacement. Compared with the use of only the FPB, the combined system may increase the energy input to the structure, thus resulting in an intensified seismic response.