In the present paper, to investigate the unsteady wake flow and its evolution at different moments when the blade sweeps the volute tongue, numerical simulation method is applied to discuss the flow field in detail based on the DDES (Delayed Detached Eddy Simulation) model when the model pump works at the nominal flow rate. Emphasis is attracted on the relative velocity distribution , besides the typical vorticity and TKE distributions at various moments are also analyzed to obtain the turbulent characteristics of the model pump. Results show that the current method has the ability to capture the main flow structure of the model pump, especially for the jet flow. From the blade pressure side to the suction side , the relative velocity distribution shows typical jet-wake flow pattern. With the impeller rotating, the reflection point of the jet-wake flow pattern changes at the mid span of the impeller . From flow distributions on different spans of the impeller, it is noted that the relative velocity decreases from the front chamber to the back chamber at major region of the blade channel. In the model pump, it is observed that several typical high vorticity regions are generated, where the high TKE values are also expected, especially at the blade trailing edge and in the volute zone caused by the wake flow. So it is inferred that for the unsteady flow in the pump and the corresponding induced pressure pulsation , controlling the wake flow is an effective approach to lower the unsteady flow pulsation in the centrifugal pump .