Objective: To evaluate the effect of backflow on blood washout performance in typical flow stagnation and recirculation regions and to elucidate its underlying mechanism, thereby providing a theoretical basis for the hemodynamic optimization of medical devices. Methods: The FDA nozzle model was employed as the investigation platform. Three scenarios: steady flow (S1), moderate pulsatile flow (S2), and high-amplitude pulsatile flow inducing reverse flow (S3), were generated via sinusoidal modulation of the inlet pressure. Washout performance was characterized using a passive scalar transport approach, and washout efficiency was quantitatively evaluated by the local and global volume fractions of old blood. Results: The decay of the global old blood volume fraction ? ˉ in S1 and S2 exhibited nearly identical trends: at flush volumes of 3.46 and 3.38, the corresponding ? ˉ values were 16.55% and 16.52%, respectively. This similarity is attributed to the inability of S2 to disrupt the stable stagnation/recirculation structure at the sudden-expansion corner. In contrast, the transient backflow induced by S3 significantly disturbed and disrupted this stagnation/recirculation structure, promoting old blood redistribution during the reverse-flow phase and facilitating its rapid washout during the subsequent forward acceleration phase. Compared with S1 and S2, S3 required the smallest flush volume (2.88) to achieve the lowest ? ˉ (6.25%), demonstrating superior washout efficiency. Conclusion: Backflow can effectively enhance washout efficiency in flow-retentive regions through a synergistic "reverse-flow disturbance and forward-flow evacuation" mechanism. This mechanism offers a novel design paradigm for flow modulation and hemodynamic optimization in blood-contacting medical devices.