<p>Efficient mixing in microfluidic channels is hindered by the small diffusion coefficient. In this study, we exploit surface acoustic wave (SAW) induced microstreaming to achieve rapid mixing of two co-flowing miscible streams within a microchannel. Three SAW actuation strategies are introduced and systematically investigated: one-side standing SAW (bottom-SSAW), two-side standing SAW, and two-side travelling SAW. To examine the first-order acoustic field, the second-order streaming flow, and the mixing performance under different SAW configurations, we develop a 3-D computational model based on the finite element method (FEM), which solves the periodic acoustic and time-averaged Rayleigh streaming fields in the frequency domain via the perturbation method. The resulting flow is then coupled with a convection-diffusion model for investigating species transport characteristics. The results show that the two-side standing SAW setup yields the strongest streaming, forming a vertically and laterally symmetric streaming field in a cross section with four bulk vortices, while the two-side travelling SAW setup introduces a centrally patterned, dominant streaming vortex that provides efficient transverse advection despite a slightly lower streaming velocity. In contrast, the conventional bottom-SSAW configuration produces weak, asymmetric streaming. For mixing applications, both two-side SAW configurations demonstrate outstanding performance with over 90% mixing efficiency at a flow rate up to ∼1 µL/min, significantly outperforming the bottom-SSAW configuration (∼60% at 1 µL/min). Our findings provide guidelines for the design of high-throughput, efficient, and versatile acoustofluidic micromixers.</p>