In　this　paper　the　research　of　mode　localization　and　transition　is　developed　in　the　field　of　space　structures.　Through　the　modal　analysis　of　a　plane　substructure　of　space　structures,　the　existence　of　mode　localization　and　transition　phenomena　is　verified.　The　mechanics　of　two　phenomena　is　disclosed　by　the　matrix　perturbation　analysis.　The　conclusion　is　that　the　internal　cause　of　two　phenomena　is　the　intensive　frequencies　of　space　structures　and　the　external　cause　is　the　change　of　structural　physical　parameters.　At　last,　the　important　impact　of　two　phenomena　was　expounded　in　the　areas　of　seismic　design　and　damage　identification　of　space　structures,　and　these　phenomena　are　suggested　to　be　considered　adequately.