An offshore long-span continuous rigid-frame bridge is taken as an example to study the effect of degradation of bond-slip behavior on the seismic performance of bridges in an offshore environment during a service period. On the basis of a numerical simulation analysis using the OpenSeeS platform, the influence of durability degradation of concrete carbonization, steel corrosion, and degradation of bond-slip performance is considered collectively using incremental dynamic analysis method to examine the time-varying seismic fragility of the offshore bridge. Results show that when bond slip is considered, the exceedance probability of the bridge components and the system increases significantly, and the durability degradation caused by concrete carbonization and chloride ion erosion in the whole life cycle increases the seismic response of the bridge structure. The results of the proposed time-varying seismic fragility analysis indicate that, considering the degradation of bond-slip behavior of reinforced concrete after the durability degradation of materials, the exceedance probability of the pier, bearing, abutment, and system increases with the extension of service period and the increase in seismic strength under earthquake action. In addition, with the extension of service time, the effect of bond slip on the seismic fragility of components and system gradually decreases.