Distributed　deformation　based　on　fiber　Bragg　grating　sensors　or　other　kinds　of　strain　sensors　can　be　used　to　monitor　bridges　during　operation.　However,　most　research　on　distributed　deformation　monitoring　has　focused　on　solid　rectangular　beams　rather　than　box　girders—a　kind　of　typical　hollow　beam　widely　employed　in　actual　bridges.　The　deformation　of　a　single-cell　box　girder　contains　bending　deflection　and　also　two　additional　deformations　respectively　caused　by　shear　lag　and　shearing　action.　This　paper　revises　the　improved　conjugated　beam　method　(ICBM)　based　on　the　long-gage　fiber　Bragg　grating　(LFBG)　sensors　to　satisfy　the　requirements　for　monitoring　the　two　additional　deformations　in　a　single-cell　box　girder.　This　paper　also　proposes　a　suitable　LFBG　sensor　placement　in　a　box　girder　to　overcome　the　influence　of　strain　fluctuation　on　the　flange　caused　by　the　shear　lag　effect.　Results　from　numerical　simulations　show　that　the　theoretical　monitoring　errors　of　the　revised　ICBM　are　typically　0.3–1.5%,　and　the　maximum　error　is　2.4%.　A　loading　experiment　for　a　single-cell　box　gilder　monitored　by　LFBG　sensors　shows　that　most　of　the　practical　monitoring　errors　are　6–8%　and　the　maximum　error　is　11%.　©　2018　by　the　authors.　Licensee　MDPI,　Basel,　Switzerland.