Due　to　the　lack　of　analytic　technique　for　simulating　the　vortex-induced　vibration　(VIV)　of　long　span　bridges,　a　combination　of　the　VIV　semi-empirical　model　with　the　structural　equation　of　motion　is　widely　employed　to　calculate　the　responses　of　bridge　structures.　However,　the　applicability　of　this　method　has　seldom　been　investigated　before.　In　this　study,　the　theoretical　defects　of　the　conventional　combination　strategy　(i.e.　the　finite　element　procedure　or　the　linear　normal　mode　procedure,　LNM)　are　first　discussed,　a　more　theoretically　reliable　approach　(the　nonlinear　norm　mode　approach,　NNM)　is　then　proposed,　and　the　closed-form　expression　for　the　NNM　of　the　VIV　system　is　derived.　The　accuracy　of　the　proposed　method　is　further　illustrated　by　two　case　studies.　This　new　approach　offers　a　theoretically　reliable　tool　for　analyzing　the　VIV　of　long-span　bridges.　It　can　also　be　applied　to　the　process　of　VIV　fatigue　analysis　or　control　strategy　optimization.