To　enhance　the　efficiency　and　stable　operation,　the　unsteady　pressure　fluctuations　in　a　centrifugal　pump　and　the　radial　force　on　an　impeller　are　investigated　for　three　different　vaned　diffuser　outlet　diameters.　The　steady-state　hydrodynamic　performance　of　the　centrifugal　pump　with　three　different　vaned　diffuser　outlet　diameters　was　experimentally　measured.　Numerical　simulations　were　used　to　obtain　the　hydrodynamic　performance　of　the　experimental　centrifugal　pump　based　on　the　Reynolds-averaged　Navier-Stokes　(RANS)　and　turbulence　models,　The　numerical　results　of　the　hydrodynamic　performance　were　in　agreement　with　the　experimental　data.　The　accuracy　of　the　utilized　numerical　approach　was　demonstrated.　The　unsteady　flow　characteristics　of　the　centrifugal　pump　were　numerically　studied.　With　increasing　diffuser　vane　outlet　diameter,　the　flow　field　within　the　volute　became　more　non-uniform,　and　the　pressure　fluctuation　was　more　drastic.　Moreover,　because　of　the　influence　of　the　non-uniform　flow　field　and　the　pressure　fluctuation,　the　radial　force　on　the　impeller　increased.