The　present　understanding　of　the　unsteady　interaction　mechanism　between　inlet　guide　vane　(IGV)　wake　with　centrifugal　compressor　impeller　and　its　vaned　diffuser　is　quite　rough.　We　carry　out　investigation　on　the　motion　and　distortion　mechanism　of　the　IGV　wake　and　large　size　vortice　in　the　centrifugal　compressor　impeller　and　its　vaned　diffuser　for　the　improvement　of　the　reliability　and　aerodynamic　performance　of　the　centrifugal　compressor.　Numerical　simulation　of　the　unsteady　IGV-impeller-diffuser　interaction　in　a　centrifugal　compressor　is　performed.　The　mechanism　of　the　unsteady　IGV-impeller-diffuser　interaction　is　explored.　Results　show　that　the　minimum　unsteadiness　in　impeller　occurs　to　the　middle　part　of　the　blade,　it　is　due　to　the　dual　unsteady　influence　of　the　IGV　wake,　large　size　vortice　behind　IGV　and　the　potential　repercussion　of　the　diffuser　on　the　impeller;　and　the　maximum　unsteadiness　appears　at　the　hub　of　the　interface　between　the　impeller　and　vaned　diffuser.　When　the　IGV　prewhirl　occurs,　the　unsteady　distribution　of　variation　is　uniform　along　the　diffuser　vane　due　to　the　simultaneous　influence　of　IGV　and　impeller　blade　wake.