The　unsteady　IGV-impeller-diffuser　interaction　in　a　centrifugal　compressor　is　investigated　numerically　and　experimentally　under　at　different　IGV　prewhirl　angle;　and　the　mechanism　of　unsteady　IGV-impeller-diffuser　interaction　is　explored.　Results　show　that　the　maximum　unsteadiness　appears　at　the　interface　between　impeller　and　vaned　diffuser.　When　the　IGV　prewhirl　occurs,　the　basic　frequency　of　the　unsteady　flow　in　the　vaneless　diffuser　is　demonstrated　as　the　blade　passing　frequency　of　the　impeller　or　the　half;　when　the　stagger　angle　of　the　diffuser　is　increased,　the　basic　frequency　is　dominated　by　the　half　of　the　blade　passing　frequency.　The　turbulence　intensity　and　unsteady　intensity　increase　toward　the　hub,　and　tend　violently　when　IGV　prewhirl　angle　of　60°　matches　with　the　diffuser　stagger　angle　of　17°.