The　purpose　of　this　study　is　to　provide　some　insights　into　the　phase　mechanism　of　a　cylindrical　vortex-induced　vibration.　A　transient　coupled　fluid-structure　interaction　numerical　model　is　adopted　to　simulate　a　cylindrical　vortex-induced　vibration.　The　vortex　shedding　around　the　cylinder　is　investigated　numerically　by　a　two-dimensional　large　eddy　simulation　approach　which　can　catch　more　details　of　the　flow　field　and　more　accuracy　on　computing　hydrodynamic　forces.　The　vortex　shedding　modes　and　response　and　hydrodynamic　forces　of　a　cylindrical　vortex-induced　vibration　are　acquired　with　varied　frequency　ratios.　According　to　differences　in　the　vortex　shedding　location,　the　vortex　wake　can　be　characterized　by　two　kinds　of　mode,　the　＂first　mode＂　and　the　＂second　mode.＂　The　mechanisms　behind　the　phases　of　the　first　mode　and　the　second　mode　vortex　wakes　are　investigated,　and　it　is　found　that　the　flow　speed　induced　by　a　cylindrical　transverse　vibration　and　the　position　of　a　vortex　release　are　the　root　causes　of　the　phase　difference　between　the　lift　coefficient　and　transverse　displacement.　The　speeds　caused　by　a　cylinder　vibration　and　a　cylinder-shed　vortex　are　the　reasons　that　the　lift　amplitude　of　an　oscillatory　cylinder　is　different　from　that　of　a　fixed　cylinder.