High　field　superconducting　magnets　are　subjected　to　a　large　Lorenz　force　during　operation.　The　main　superconductors　for　high　field　applications　like　Nb3Sn　are　strain　sensitive,　therefore　it　is　essential　to　measure　the　strain　in　the　superconducting　(SC)　coils　during　the　testing　process.　Compared　with　the　traditional　way　by　using　the　Resistance　Strain　Sensor　(RSS),　the　fiber　optic　sensor　based　on　the　Fiber　Bragg　grating　(FBG)　method　has　significant　advantages　in　high　field　applications.　The　FBG　is　an　anti-electromagnetic　interference　device,　which　makes　it　possible　to　measure　the　local　strain　inside　the　superconducting　coils.　Meanwhile,　it　is　sensitive　only　to　the　strain　at　a　constant　low　temperature　(4.2　K).　In　this　study,　several　FBGs　and　RSSs　were　impregnated　in　SC　coils　of　a　12　Tesla　dipole　magnet,　and　FBGs　were　attached　to　the　outer　surfaces　of　the　aluminum　shell　and　rods.　The　test　was　performed　in　the　whole　process　from　room　temperature　assembly　of　the　magnet　to　the　excitation　at　4.2　K.　The　experimental　results　show　that　FBGs　can　continually　monitor　the　strain　of　the　magnet　from　a　macro　perspective,　as　well　as　reflect　the　internal　strain　state　of　the　SC　coils　during　the　whole　process.　Compared　with　RSSs,　FBGs　have　a　better　performance　in　strain　measurement　during　the　excitation　due　to　the　anti-electromagnetic　interference.　Strain　measurement　inside　the　magnet　can　offer　more　useful　information　about　the　strain　distribution　of　the　magnet　and　the　origin　of　the　quench,　which　may　help　developers　improve　the　design　and　fabrication　of　SC　magnets.