A　vibration　isolation　structure　is　composed　of　a　superstructure　and　a　vibration　isolation　layer　including　vibration　isolation　bearings　and　dampers　with　different　damping　features　from　those　of　superstructure,　which　possess　typically　non-proportional　damping　features.　So,　the　damping　matrix　of　this　system　can＇t　be　decomposed　via　the　system＇s　undamped　modal　shapes　and　the　traditional　modal　shapes　superposition　response　spectrum　method　is　not　applicable　to　this　system.　Here,　based　on　the　random　vibration　theory　and　considering　features　of　vibration　isolation　structures,　a　multi-dimensional　earthquake　complex　modal　shapes　superposition　response　spectrum　method　was　proposed,　it　could　consider　non-proportional　damping　features.　The　error　of　the　forced　decoupling　method,　an　approximate　approach　commonly　used,　was　studied.　It　was　found　that　the　energy　transfer　between　the　superstructure　and　the　vibration　isolation　layer　is　prevented　with　this　method　to　cause　smaller　seismic　responses　of　the　superstructure.　The　vibration　isolation　benchmark　model　was　taken　as　an　example　to　implement　the　time-history　method,　the　forced　decoupling　method　and　the　proposed　complex　modal　shapes　superposition　response　spectrum　method,　respectively.　Three　results　were　compared,　it　was　shown　that　the　proposed　method　has　a　better　accuracy　and　can　fully　reflect　the　non-proportional　damping　characteristics　in　vibration　isolation　systems;　when　the　damping　of　the　vibration　isolation　layer　is　larger,　the　forced　decoupling　approach　has　a　worse　accuracy,　it　can＇t　reflect　the　amplification　effects　of　the　superstructure　seismic　responses　due　to　damping　of　the　vibration　isolation　layer.　©　2017,　Editorial　Office　of　Journal　of　Vibration　and　Shock.　All　right　reserved.