This　paper　presents　a　measurement　technology　based　on　the　multi-element　fiber　Bragg　grating　(FBG)　sensors　for　monitoring　the　deflection　of　thin-walled　wing　spar.　A　finite　element　model　of　the　tested　thin-walled　beam　structure　subjected　to　bending　load　is　developed　in　order　to　analyze　stress　status　and　to　derive　the　convenient　position　of　the　optical　fiber　sensors　for　a　correct　monitoring　of　the　wing　spar　beam.　FBG　rosette　has　been　designed　between　the　shear　web　and　spar　cap　as　a　new　installation　position　of　bonding　optical　fiber　sensors　for　estimation　the　deflection.　An　experimental　campaign　is　carried　out　in　order　to　verify　the　proper　behavior　of　the　deflection　of　wing　spar　beam　and　sensing　strategy.　The　loading　test　results　revealed　that　multi-element　FBG　rosette　was　successfully　implemented　on　the　tested　thin-walled　wing　spar　beam　as　well　as　providing　reliable　strain　and　deflection　monitoring　data　from　the　FBG　sensors.　The　results　show　that　a　good　correspondence　between　the　input　load　and　output　strain　of　the　FBG　sensing　system　with　good　reproducibility,　and　the　comparative　analysis　between　FBG　and　resistance　strain　gage　have　been　achieved,　which　measurement　results　of　both　are　consistent.　These　data　and　monitoring　method　are　useful　for　fatigue　life　prediction　of　the　potential　thin-walled　I-beam　structure.　(C)　2018　Elsevier　GmbH.　All　rights　reserved.