An experimental investigation based on particle image velocimetry (PIV) technology was used to measure the internal flow in a low-specific-speed centrifugal pump impeller. The main purpose of this paper is to quantitatively study the influence of the internal flow patterns on the hydraulic performance and energy conversion characteristics through this visualization experiment. The PIV measuring region covers full impeller channels. Phase-averaged relative velocity distributions were calculated to investigate the evolution of internal flow pattern with the decrease in flow rate. The effect of flow pattern on pump head has also been investigated qualitatively. Results show that a clockwise vortex on the blade suction side and a counterclockwise vortex on the blade pressure side will appear in different impeller passages with the decrease in flow rate. The counterclockwise vortex moves towards the impeller outlet, but the clockwise vortex moves to the opposite side. Detailed studies show that the motion of the vortices considerably impacts pump hydraulic performance. The counterclockwise vortex will increase energy losses, whereas the clockwise vortex positively affects the pump head. Moreover, the blade loading distribution and its variation with flow rate are proposed to analyse the energy conversion mechanism in the centrifugal pump impeller. (C) 2019 Elsevier Ltd. All rights reserved.