Earthquake load has the character of random. The uses of anti-seismic measures have been proved to be effective on the reduction of bridge seismic damage when the bridges encounter with rare earthquake or unexpected earthquake. Unseating prevention devices are the ultimate security installment in the collapse-proof system, which used to protect the substructure falling down from piers as the supports lose their effectiveness. Based on relational design standards at home and abroad, time-history analysis has been conducted on the structure with cable-unseating prevention devices under rare earthquake and strong earthquake sequence. The variation laws involving the collapse-proof result of unseating prevention devices and effect of collapse-proof changing with both cable length and designed value for displacement have been obtained. Under rare earthquake, the rational unseating prevention devices could provide the ultimate security to the structure, increase the integrity and anti-pushing rigidity of the structure, and make the seismic load to be well-distributed among the piers. The unseating prevention devices are able to provide the ultimate security for the damaging structure under the strong earthquake sequence; and achieve the request that there is no collapse when strong earthquake occurs.