For　the　safety　evaluation　of　narrowband　used　in　engineering　structures,　the　propagation　characteristics　of　guided　waves　in　a　narrowband　are　investigated　experimentally.　Firstly,　the　characteristics　of　guided　waves　propagated　in　an　infinite　plate　is　theoretical　analyzed;　then　the　contrastive　experiments　are　conducted　in　narrowbands　with　different　widths　using　different　modes　of　guided　waves.　It　is　found　that　comparing　to　the　lamb　modes,　shear　horizontal　waves　(SH0)　are　almost　not　affected　by　the　boundary　of　a　narrowband,　and　the　width　variation　of　a　narrowband　has　no　influence　on　the　waveforms　of　an　SH0　mode　when　the　width　of　a　narrowband　is　bigger　than　the　width　of　electromagnetic　acoustic　transducers.　Finally,　the　SH0　mode　guided　wave　is　used　to　the　non-destructive　testing　(NDT)　of　flat　steel　in　a　ground　grid.　It　is　showed　that　a　SH0　mode　can　propagate　a　long　distance　in　flat　steel,　and　can　be　used　for　NDT　of　buried　flat　steel.　The　developed　method　provides　feasible　scheme　for　the　health　monitoring　of　narrowbands　in　engineering　structures.