Earthquake has the character of cluster. The aftershocks happen after the mainshock and the mainshock with great magnitude has larger destructive aftershocks. As an important part of lifeline engineering, bridges are re-quired to be secure during earthquake. Using the Chi-Chi earthquake sequence, the time-history analysis has been carried out for bridge; the girder will fall down from supports under mainshock, and may fall off the piers under af-tershocks. Based on the related design standards at home and abroad, we designed the cable-unseating prevention devices for the bridge. The change of the structural seismic responses and the effect of collapse-resistance were compared. Unseating prevention devices can provide the ultimate security to the structure under earthquake, ensure the integrity and increase the anti-pushing rigidity of the structure. As the load of side-pier is increased, the plastic deformation occurs under mainshock, but the bridge is secure under aftershocks. It's necessary to analyze the seis-mic performance of bridges under earthquake sequences.