In order to study the pressure fluctuation characteristics of a pump-turbine under the abnormally low head, the geometric model of a pump-turbine is investigated using Realizable k-ɛ turbulence model. The unsteady numerical simulation of four operating points is conducted. The pressure signals of the points are achieved. The CFD results show good agreements with experimental ones. And then, the pressure signals under the operating condition a₀ = 37 mm and Q₁₁ = 0.865 m³/s are analyzed in the time domain and frequency domain respectively. It is concluded that the pressure fluctuation changes in periodic variation at the vaneless space between the guide vane and the runner and its frequency is the frequency of blade passing through. The pressure fluctuation inside the bent pipe is low-frequency, whose frequency is the 0.34 - 0.35 times of the frequency of the runner passing through.