Abstract: Particle image velocimetry(PIV) system,pump experimental facility and test scheme were introduced.An experimental investigation of pump sump internal flow was performed at five different flow rates by means of PIV.The velocity vector distribution was obtained,and the characteristic parameters on the outlet section and the instantaneous volume flow rate on the inlet section were calculated.The results indicate that,the velocity distribution is uniform in the inlet and non-uniform in the outlet,and biased flow phenomenon appears in the outlet of the pump sump.Under the scope of the investigation,axial velocity distribution uniformity on the outlet section is improved and the velocity-weighed average bias angle decreases with the increase of flow rate.The instantaneous volume flow rate is impressively pulsatile and presents the Gaussian distribution,and the relative pulsation amplitude is within 6%.The mean flow rate in a longer period agrees with the flow rate measured by the turbine flowmeter.This confirms that the PIV system can be applied to measure the instantaneous volume flow rate and investigate the unsteady characteristics.