This paper presents an experimental investigation of flow field instabilities in a centrifugal pump impeller at low flow rates. The measurements of pump hydraulic performance and flow field in the impeller passages were made with a hydraulic test rig. Analysis of Q-ΔP-η data and flow structures in the impeller passages were performed. In the present work, the effect of various flowrates on centrifugal pump impeller performance was analyzed based on pump measured parameters. The impeller’s geometry was modified, with positioning the curved spacer at the impeller suction side. This research investigates the effect of each inlet curved spacer model on pump performance improvement. The hydraulic performance and cavitation performance of the pump have been tested experimentally. The flow field inside a centrifugal pump is known to be fully turbulent, three dimensional and unsteady with recirculation flows and separation at its inlet and exit. The 3-D turbulent flow inside the pump with different curved spacers were analyzed by using a three-dimensional Navier-Stokes code with a standard k-ε turbulence model. In order to improve the pump impeller, the successful curved spacer model based on CFD analysis was manufactured and positioned at the inlet section of reference impeller for testing and analysis. Based on the numerical simulation and experimental tests, the following conclusions can be drawn: (1) The impeller inlet geometry has important influence on the performance improvement of a centrifugal pump. Favorable effects on performance improvement have been achieved by separating the inlet flow region into two lanes. (2) Improvement on inlet static and total pressure values were achieved, mostly at low flowrate trials. (3) A uniform flow upstream of the impeller inlet is helpful for improving cavitation inside the pump. (4) The experimental results provided important evidence for the influence of flow instabilities on pump hydraulic performance.