Abstract: In order to clarify the evolution of the vortex structure in the impeller of a centrifugal pump as a turbine and the unsteady flow characteristics under low-flow conditions, this paper adopts the Omega vortex identification method and the dynamic mode decomposition (DMD) method to study the unsteady flow field of the impeller of a single-stage centrifugal pump as a turbine under low-flow condition. The results show that the vortex structure inside the turbine impeller under low-flow condition can be better identified based on the Omega vortex identification method. The flow inside the turbine impeller under low-flow condition is complex, with mainly large-scale elongated vortex flow, locally small-scale tubular vortex flow, and multi-scale vortexes periodically merging, separating and colliding, which leads to kinetic energy loss. Under the 0.6Q_d condition, the flow in the impeller channel is complex, there are more vortex structures, and the impeller vortex accounts for most of the area of the impeller channel, which has an important effect on the performance of the pump as turbine. The vortex structure in the impeller channel is more turbulent, and there are many small vortex structures in the flow channel near the impeller outlet. The fluid flows into the impeller from the worm shell, impacts the blade, and forms a small-scale vortex near the inlet of the impeller channel, and the fluid flows out along the impeller channel to the outlet of the impeller channel; most of the high-flow velocity region is distributed near the inlet of the impeller channel near the back of the blade, with a maximum flow velocity of 10.87 m/s, and the low-flow velocity region is basically distributed in the middle of the flow channel near the working surface of the blade, with a minimum flow velocity of less than 0.1 m/s. The DMD method can effectively identify the pulsation frequency of the complex flow in the impeller under the low-flow condition, and the decomposition can obtain the first four main modes of the flow field and their related frequency information, which are divided into static and dynamic interference modes, fundamental modes and dissipative modes, and can clearly reflect the complex flow characteristics in the impeller under the small flow condition. According to the size of the energy of the modes, the top 4 modes are selected, and the one with the highest energy is labelled as the 1st-order mode. The energy of the 1st-order mode is 109,750, which accounts for 75.4% of the energy of the whole flow field, indicating that the 1st-order modes make the main contribution to the whole flow field and dominate the field. The 1st order modes are static and dynamic interference modes, the 2nd order modes are fundamental modes with a frequency of 0, which represent the basic steady state structure and characterize the flow field caused by the geometry of the flow channel, and the 3rd and 4th order modes are the high harmonic behaviours of the static and dynamic interference modes, which characterize the static and dynamic interference effect of the impeller rotating on the flow field, and the irregular coherent structure appears in the impeller channel in the 3rd and 4th order modes, which reflect the unstable fluid mass fragmentation and dissipation characteristics in the impeller channel. This reflects the characteristics of unstable fluid mass fragmentation and dissipation in the impeller channel. In this paper, we investigate the space-time evolution law of the unsteady vortex structure inside the pump as turbine in the region of low-flow condition, and the distribution of the coherent structure in each mode, the results of the study may provide a basis for widening the high efficiency zone of the pump as turbine.