The characteristics of pressure fluctuations in a prototype reversible pump turbine is investigated within a wide range of load conditions with a focus on the low load condition (e.g. 25% of rated power) with the aid of pressure signals obtained at several typical recording points (i.e. spiral casing, vaneless space, draft cone tube and draft elbow tube). Our findings reveal that at low load condition, the pressure fluctuation is quite significant (e.g. above 12% in terms of non-dimensional values) especially in the vaneless space and spiral casing with the dominant frequency being the blade passing frequency. Furthermore, based on the characteristics of pressure fluctuation, the investigated load range is divided into three zones. For zone I (with low load), the amplitude of pressure fluctuation is highest and the dominant mechanism is the rotor-stator interaction in the vaneless space with the blade passing frequency. For zone II (with medium load), the amplitude of pressure fluctuation is less prominent (below 5%) and the dominant mechanism is the low-frequency fluctuations induced by the swirling vortex rope. For zone III (with high load), the amplitude of pressure fluctuation is quite limited (less than 3%) and the dominant mechanism is still the rotor-stator interaction but with the dominant frequency being the harmonics of blade passing frequency. Detailed examples for all three zones are given and discussed with quantitative descriptions of propagation mechanism of fluctuation.