A　two-dimensional　frequency-domain　optical　vibration　tomography　system　(2D-FOCVT)　was　used　for　the　modal　analysis　of　added-mass　cantilever　beams　and　finally　to　do　mass　detection.　Based　on　describing　the　theory　and　method　for　the　natural　frequencies　of　lumped　mass　cantilever　beams,　the　first　three　order　natural　frequencies　of　a　cantilever　beam　with　different　added　mass　were　analyzed,　and　the　sensitivity　of　the　additional　lumped　mass　to　the　first　three　order　natural　frequencies　was　investigated.　Subsequently,　the　relationship　between　the　additional　lumped　mass　and　the　first　three　order　natural　frequencies　measured　by　the　home-made　2D-FOCVT　system　was　achieved　by　using　the　numerical　fitting　method.　The　variation　of　the　sensitivity　along　with　the　change　of　the　additional　lumped　mass　was　discussed.　On　this　basis,　a　mass　weighing　method　using　the　sensitivity　of　the　first　order　natural　frequency　was　proposed　by　virtue　of　the　2D-FOCVT　system.　The　experimental　results　show　that　the　home-made　2D-FOCVT　system　exhibits　a　good　performance　with　nano-scale　high　resolution;　also,　it　is　a　novel　application　of　2D-FOCVT　in　the　modal　analysis　for　mass　weighing.　©　2020,　Editorial　Office　of　Journal　of　Vibration　and　Shock.　All　right　reserved.