Hysteresis loops on the performance curves in the hump region of pump-turbines have been identified in previous studies, which significantly enlarged the unstable operational region, leading to unreasonable selections in the safety margin in the head. In order to understand the formation mechanism of the hysteresis characteristic, and its dependence on the guide vane opening, experiments on both the performance characteristics and the pressure fluctuations were carried out in a low specific speed (n(q) = 36.1 min(-1)) model pump-turbine. Hysteresis phenomena with three guide vane openings (13 mm, 19 mm and 25 mm) were confirmed. Analyses through the Euler theory indicate that all hysteresis characteristics are originated from combining effects from the decrease in the Euler head and the increase in the hydraulic loss. Detailed analysis reveals that the hydraulic loss contributes the most to the generation of the hysteresis characteristic. Based the analysis of pressure fluctuations, it is obtained that the high hydraulic loss is related to the low frequency pressure fluctuations, originating from different sources under different guide vane openings. At small guide vane openings, low frequency components (between 0.04 and 0.15 times rotational frequency) originate from rotating flow separation in the vaneless space, while at large guide vane openings, they result from complex vortices in the guide vanes. Furthermore, different initial operating conditions lead to different magnitudes of low frequency components, which in turn contribute to the hysteresis characteristic. The present study provides a sound criterion for the selection of the safety margin in the head, with respect to the hump characteristic of pump-turbines in engineering applications. (C) 2019 Elsevier Ltd. All rights reserved.