In the present paper, to investigate unsteady flow structures and pressure pulsations at off-design conditions, a low specific speed centrifugal pump is investigated by the DDES (Delayed Detached Eddy Simulation) method. Emphasis is laid on the unsteady evolution of flow structure and pressure pulsation at the stalled condition. Results show that the jet-wake flow pattern could be captured by the current numerical simulation method, and it shows a high calculation precision for the jet flow. Relative velocity decreases rapidly in the wake region towards the blade suction side. Significant inflection point exists between the jet and wake flow regions, and it almost keeps unchanged at the identical impeller-volute position under various flow rates. At the stalled status, the inflection point moves towards the blade pressure side with the blade passing the volute tongue, and evident non-uniform flow distributions are formed in the blade channels characterized by the multiple vortices. The blade channel is gradually blocked by the corresponding stall cell when the blade approaches the volute tongue, and it indicates that the volute tongue has a significant effect on the evolution process of the stall structure. By combination analysis of the unsteady evolution of the stall structure and pressure spectrum, the stall frequency at 0.25f(n) (being f(n) the impeller rotating frequency) is captured for the current model pump. (c) 2020 Elsevier Ltd. All rights reserved.