The　terminal　building　of　Beijing　Capital　Airport　(T3),　a　super　large　long-span　complex　spacial　steel　structure,　is　made　up　of　steel　frame　and　double-layer　reticulated　shell,　which　is　960m　long　and　780m　wide.　Response　spectrum　analysis　is　conducted　for　it　by　using　SAP2000　finite　element　software.　The　influences　of　one-dimensional　and　multi-dimensional　earthquake　input　on　the　element　force,　the　joint　displacement　and　the　total　earthquake　shearing　force　are　analyzed.　The　relationship　between　seismic　response　under　multi-dimensional　earthquake　input　and　that　calculated　by　Chinese　Earthquake　Criterion　combination　formula　is　studied.　The　relationship　between　the　number　of　modes　and　modal　participating　mass　ratio,　and　the　relationship　between　the　number　of　modes　used　in　CQC　method　and　the　seismic　response　of　structure　are　investigated.　These　researches　show　that　the　influence　of　high　frequency　modes　on　the　structure　internal　force　is　little　and　could　be　ignored.　According　to　the　comparison　between　the　seismic　response　under　one-dimensional　and　multi-dimensional　earthquake　input,　the　response　spectrum　analysis　under　multi-dimensional　earthquake　input　should　be　conducted　for　the　super　large　long-span　complex　spacial　steel　structure.　In　addition,　this　paper　provides　a　new　method　of　earthquake　combination,　which　can　substitute　the　multi-dimensional　earthquake　analysis　and　make　up　the　deficiencies　of　the　current　specifications.