Based　on　the　theory　of　elastic-plastic　response　spectrum　and　the　relationship　between　displacement　ductility　and　equivalent　damping　ratio　of　a　non-linear　restoring　force　model,　according　to　the　results　of　a　single-mass　elasto-plastic　response　analysis,　the　elasto-plastic　response　spectrum　of　an　isolated　structure　was　established.　Further,　compared　with　the　standard　response　spectrum　of　our　country,　the　results　showed　that　the　values　of　the　elasto-plastic　acceleration　response　spectra　in　the　long　period　cycles　are　overhigh,　damping　attenuation　coefficients　are　also　larger.　Based　on　the　results　of　a　single-mass　elasto-plastic　seismic　analysis,　the　elasto-plastic　response　spectrum　formula　of　isolated　structures　was　established.　At　last,　shaking　table　tests　of　a　5-story　steel　isolated　structure,　the　earthquake　responses　of　the　isolated　structure　using　the　new　suggested　acceleration　response　equation　and　the　time　history　analysis　method　were　compared,　and　the　results　obtained　by　using　the　modified　long　period　response　spectrum　were　consist　with　those　from　time　history　analyses　and　tests;　the　results　by　using　the　proposed　equation　of　response　spectrum　had　a　higher　precesion　for　isolated　structures.