[Objective] The steel volute of a pumped storage power station is the part of the flow passage structure subjected to the highest internal pressure, bearing cyclic water pressure during operation and facing potential risk of low-cycle fatigue failure. At present, a fundamental issue in predicting low-cycle fatigue life of steel volutes in pumped storage power stations lies in the scientific determination and input of the fatigue load spectrum. [Methods] Static analysis of the composite structure was performed on the Abaqus finite element platform. Based on water level monitoring data, static monitoring results, and unit operating modes, the prototype load spectrum, rainflow-counting load spectrum, and constant-amplitude load spectrum were respectively compiled. [Results] By comparing with the fatigue life prediction results based on the prototype load spectrum, the reliability of the load spectrum compiled by the rainflow-counting method was verified. The sequence of cycle amplitudes and extremely small amplitude loads in rainflow counting had minimal impact on fatigue life prediction results, indicating that the rainflow-counting method could serve as a simplified input approach for low-cycle fatigue loading of steel volutes. The fatigue life prediction results based on the constant-amplitude load spectrum were close to those based on the prototype spectrum. For this pumped storage power station, constant-amplitude loading could be used as a simplified input for fatigue loading. However, it should be noted that the drawdown depth of the water level at this power station was relatively small. In such cases, whether the prototype load spectrum, rainflow-counting load spectrum, or constant-amplitude load spectrum was used, the range of cyclic amplitude variation was limited. For power stations with small water level drawdown depths, the fatigue load spectrum of the steel volute could be simplified to a constant-amplitude form. However, for power stations with relatively large water level drawdown depths, whether the above conclusions were applicable required further investigation. [Conclusion] The findings of this study can provide a reference for compiling low-cycle fatigue load spectra for steel volutes. According to the prediction results, there is no risk of low-cycle fatigue failure during the operation period of the power station. However, certain limitations in the calculations of this study may lead to an overestimation of prediction results for the following three reasons. (1) Only the impact of hydrostatic pressure on the steel volute is considered, while variations in water hammer pressure during transitions of unit operational states are not taken into account. (2) Seasonal variations in water temperature inside the steel volute significantly affect the timing and spatial distribution of contact closure between the steel volute and concrete, and neglecting temperature effects may underestimate the stress level in the steel volute. (3) During operation, the steel volute and concrete jointly bear the internal water pressure. Cracking in the concrete weakens its restraining effect on the steel volute, leading to an underestimation of the stress level in the steel volute in calculations.