In　order　to　explore　the　mechanism　during　the　process　of　the　non-synchronous　vibration　(NSV),　the　flow　field　formation　development　is　investigated　in　this　paper.　Based　on　the　fluid-structure　interaction　method,　the　vibration　of　rotor　blades　is　found　to　be　in　the　first　bending　mode　with　a　non-integral　order　(4.6)　of　the　rotation　speed.　Referring　to　the　constant　inter　blade　phase　angle　(IBPA),　the　appearances　of　frequency-locking　and　phase-locking　can　be　identified　for　the　NSV.　A　periodical　instability　flow　emerges　in　the　tip　region　with　the　mixture　of　separation　vortex　and　tip　leakage　flow.　Due　to　the　nonlinearities　of　fluid　and　structure,　the　blade　vibration　exhibits　a　limit　cycle　oscillation　(LCO)　response.　The　separation　vortex　presenting　a　spiral　structure　propagates　in　the　annulus,　indicating　a　pattern　as　modal　oscillation.　A　flow　induced　vibration　is　initiated　by　the　spiral　vortex　in　the　tip.　The　large　pressure　oscillation　caused　by　the　movement　of　the　spiral　vortex　is　regarded　as　a　main　factor　for　the　presented　NSV.　As　the　oscillation　of　blade　loading　occurs　with　blade　rotating　pass　the　disturbances,　the　intensity　of　the　reverse　leakage　flow　in　adjacent　channels　also　plays　a　crucial　role　in　the　blade　vibration.