Mode　localization　and　transition　in　spatial　structures　are　studied　in　this　paper.　Through　the　modal　analysis　of　a　plane　substructure　of　spatial　structures,　the　existence　of　mode　localization　and　transition　is　verified.　The　mechanism　of　two　phenomena　is　rerealed　by　the　perturbation　theory.　The　conclusion　is　that　the　internal　cause　of　two　phenomena　is　intensive　frequencies　of　spatial　structures　and　the　external　cause　is　the　slight　change　of　structural　physics　parameters.　At　last,　the　important　impact　of　two　phenomena　is　expounded　in　the　areas　of　earthquake　resistance　calculation　and　damage　identification　of　spatial　structures,　and　these　phenomena　are　suggested　to　be　considered　adequately.　It　is　showed　that　the　earthquake　resistance　calculation　without　considering　mode　localization　may　be　unsafe.　And　it　may　be　an　effectual　approach　to　solve　the　problem　of　damage　identification　of　spatial　structure　by　mastering　and　using　mode　localization　and　transition　phenomena.