Thispaperpresentsanimprovedunstructuredgridimmersedboundarymethod.Theadvantagesofbothimmersedboundarymethodandbodyfittedgridswhicharegeneratedbyunstructuredgridtechnologyareusedtoenhancethecomputationefficiencyoffluidstructureinteractionincomplexdomain.TheNavier-StokesequationwasdiscretizedspaciallywithcollocatedfinitevolumemethodandEulerimplicitmethodintimedomain.TherigidbodymotionwassimulatedbyimmersedboundarymethodinwhichthefluidandrigidbodyinterfaceinteractionwasdealtwithVOS(volumeofsolid)method.AnewVOScalculationmethodbasedongraphwaspresentedinwhichbothimmersedboundarypointsandcrosspointswerecollectedinarbitraryordertoformagraph.Themethodisverifiedwithflowpastoscillatingcylinder.
