This work investigates particle focusing under Dean-flow-coupled elasto-inertial effects in symmetric serpentine microchannels. A small amount of polymers were added to the sample solution to tune the fluid elasticity, and allow particles to migrate laterally and reach their equilibriums at the centerline of a symmetric serpentine channel under the synthesis effect of elastic, inertial and Dean-flow forces. First, the effects of the flow rates on particle focusing in viscoelastic fluid in serpentine channels were investigated. Then, comparisons with particle focusing in the Newtonian fluid in the serpentine channel and in the viscoelastic fluid in the straight channel were conducted. The elastic effect and the serpentine channel structure could accelerate the particle focusing as well as reduce the channel length. This focusing technique has the potential as a pre-ordering unit in flow cytometry for cell counting, sorting, and analysis. Moreover, focusing behaviour of Jurkat cells in the viscoelastic fluid in this serpentine channel was studied. Finally, the cell viability in the culture medium containing a dissolved polymer and after processing through the serpentine channel was tested. The polymer within this viscoelastic fluid has a negligible effect on cell viability.