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The runaway condition is unsteady and is of significant importance in pump turbines. In this research, the shear-stress transport (SST) k - ω turbulence model is used to simulate the unsteady flow in the vaneless region and the runner of a prototype pump turbine. The characteristics of pressure fluctuations, vortex identification based on Q criterion and local entropy generation rate (LEGR) in the pump turbine under runaway condition are studied, and the relationships of these physical quantities in the vaneless region are determined. Internal flow research reveals the motion and transmission characteristics of vortices in the vaneless region and the runner. The results show that the vortices that move clockwise in the vaneless region are responsible for the flow separation, the increase of LEGR, and the generation of the characteristic frequency 1.4 f n of these physical quantities (pressure fluctuations, Q criterion and LEGR). In runner, it is found that the characteristic frequency of pressure fluctuation is 3.6 f n , which satisfies a linear relationship with the characteristic frequency 1.4 f n in the vaneless region. The vortices, moving in the vaneless and runner region, interact with the blades of runner and mainly contribute to the pressure fluctuation characteristic frequency in the runner. • Unsteady flow and fluctuations of a pump turbine under runaway condition are studied. • Flow separation and vortices cause energy dissipation in vaneless region and runner. • Vortex motion greatly affects pressure fluctuation in vaneless region. • Interaction between blade and vortex causes pressure fluctuation frequency in runner. [ABSTRACT FROM AUTHOR]