A　new　seismic　isolation　system,namely　the　prestress　rubber　isolator(PRI,Chinese　patent　application　number:200910174415x),is　introduced.The　basic　principle　of　this　new　isolator　is:　making　all　or　part　of　vertical　deformation　of　isolator　be　finished　in　advance　by　exerting　a　longitudinal　pre-compressional　stress　on　the　rubber　isolator　with　flexible　cables,　the　prestress　tension　cables　become　loose　because　of　unloading　after　the　exerting　of　superstructure　loads,and　the　loose　cables　can　limit　the　horizontal　displacement　and　resist　the　tension　of　isolator.　The　calculation　formula　of　horizontal　stiffness　of　isolator　is　derivated,and　the　mechanical　properties　are　researched.　The　research　results　show　that:besides　the　horizontal　isolating　property　of　common　rubber　isolators,the　prestress　rubber　isolator　has　also　properties　of　reducing　vertical　deformation,　limiting　horizontal　displacement　and　resisting　tension.Moreover,the　prestress　isolator　has　also　a　certain　vertical　isolating　affect　because　of　the　improve　of　rubber　thickness.