In order to accurately analyze the unsteady flow characteristics in the impeller of a double-suction pump as a hydraulic turbine, the SST k-ω turbulence model was used for numerical simulation under design conditions. The dynamic mode decomposition (DMD) of a periodic unsteady velocity field was carried out, and the first four main modes and their corresponding spatio-temporal information were obtained by combining the Q criterion. The analysis results showed that the DMD method decomposed the complex flow field characteristics in the impeller into dynamic and static interference modes, basic modes and dissipative modes. Among them, the dynamic and static interference modes were dominant, and the frequency was the rotation frequency of the impeller, which reflected the flow characteristics of the flow in the impeller disturbed by the static components. The vortex structure was mainly point vortex and discontinuous tubular vortex. The basic modal frequency was zero, which reflected the steady-state flow field characteristics caused by the geometric characteristics of the blade flow channel. The vortex structure was mainly continuous tubular vortex. The third-order and fourth-order modes were dissipative modes, which reflected the characteristics of flow separation and unstable vortex structure shedding on the blade caused by the interference of static components in the impeller, mainly sheet vortex and discontinuous tubular vortex. The DMD method can decompose the flow field structure of the impeller at a specific frequency, and can clearly analyze the unsteady characteristics of the complex flow field in the impeller of the double-suction pump as a hydraulic turbine.