In this paper, continuous plasma extraction under viscoelastic fluid in a straight channel with asymmetrical expansion-contraction cavity arrays (ECCA channel) is demonstrated by exploiting the Dean-flow-coupled elasto-inertial effects. First, the forces experienced by particles in the ECCA channel were discussed. Then, 4.8 μm diameter particles, which mimic the behaviour of red blood cells (RBCs), were used to study the effects of poly(ethylene oxide) (PEO) concentrations and flow rates on particle viscoelastic focusing. Also, 3 μm, 4.8 μm and 10 μm diameter particles, which are comparable in size to platelets, RBCs, and white blood cells (WBCs), respectively, were used to study the effect of particle size on particle viscoelastic focusing. Finally, plasma extraction from diluted blood samples under viscoelastic conditions was conducted, and the purity of the collected blood plasma was measured. After two series of filtration with the same ECCA channel, the purity of 3 μm, 4.8 μm and 10 μm diameter particles reached 100%, and the plasma purity reached 99.99%, as measured by a hemocytometer. In addition, flow cytometry data further validated the filtration performance of blood plasma. By exploiting the Dean-flow-coupled elasto-inertial effects, the ECCA channel offers a continuous, sheathless, and high purity plasma extraction.