The internal characteristics during rotational speed fluctuation have an important influence on centrifugal pump to avoid or utilize its transient periformance. In this paper, a circulation piping system that includes a low-specific-speed centrifugal pump is established to study the energy distribution characteristics in a centrifugal pump during speed fluctuation. The unsteady flow in the entire system is numerically calculated with a user-defined function, the sliding grid method, and the RNG κ–ε turbulence model. Then, the energy distribution of the transient flow field in the centrifugal pump model during speed fluctuation is diagnosed with vortex dynamics by using flow section and boundary vorticity flow diagnosis methods. Results confirm that the total pressure flow energy is relatively high in the process of speed reduction because of the small hydraulic loss, and the opposite was observed in the process of speed increase. The change laws of instantaneous dimensionless head and instantaneous dimensionless flow during speed fluctuation are contrary to that of speed fluctuation. This study provides a reference for the investigation of energy distribution characteristics in the process of speed fluctuation.