In the present paper, the vibrational performance and its physical origins of a prototype reversible pump turbine in the pumped hydro energy storage power station is experimentally investigated. Specifically, the vibrations of the unit in the X, Y and Z directions of the top cover, the upper and the lower brackets were all measured for three water heads (from 48% to 90% in terms of non-dimensional values) and nine load conditions (from 34% to 96% of the rated power) together with the pressure measurement at several typical monitoring points. Based on our analysis, it was found that the vibrations of the top cover are mainly induced by the fluid flow inside the reversible pump turbine while the vibrations of the upper and the lower brackets are generated by the mechanical aspects of the rotor. For the top cover, three regions are proposed with their characteristics and its physical origins fully demonstrated with the aid of several typical examples. In region I (with the low partial loads), the vibrational level of the unit is the highest and its physical origin is the pressure fluctuation in the vaneless space with the blade passing frequency. In region II (with the medium loads), the vibrational level of the unit is medium and its physical origin is the swirling vortex in the draft tube. In region III (with the high partial load), the vibrational level of the unit is the lowest and its physical origin is twofold: the pressure fluctuation in the vaneless space with the harmonics of blade passing frequency and the mechanical aspects of the rotor. For different water heads, transitions between aforementioned categorized regions could be observed with the primary characteristics maintained. At last, comparing with the cases of the top cover, the vibrations of the upper and the lower brackets are less affected by the load and the water head variations. (C) 2018 Elsevier Ltd. All rights reserved.