As a public facility, the seismic performance of long-span bridges has been frequently studied. The seismic excitations that are inputted on the supports of long-span bridge structures are different under earthquakes; consequently, the responses of bridge structures under multistage earthquakes are different. This research analyzes the influence of the radiation wave motion around a bridge pier on the bridge structure under the action of a multilevel earthquake and discusses the pier-water coupling boundary of long-span bridges. Based on the response spectrum theory, the maximum seismic force endured by the long-span bridge structure is calculated, and the multilevel seismic response curve of the structure is obtained. Furthermore, taking a six-span bridge as an example, this research analyzes the response time histories of displacement, sheer force, and bending moment of the bridge under multistage earthquakes and presents some effective conclusions.