In this paper, we propose acoustophoretic microfluidic devices with an acoustic transparent polymer wall using a simple and low-cost fabrication method followed by MEMS (Micro-Electromechanical Systems) processes. Generally, due to the acoustic standing wave between two opposing walls in microfluidic channel, the particle focusing lines are fixed according to the applied frequency. In the proposed device, however, it is possible to place the particle focusing lines in the arbitrary position within the fluidic domain through the optimized width of polymer wall. The PDMS (Polydimethylsiloxane) mold with thin layer was used as the sealing layer between the Si substrate and cover glass, as well as the decoupling layer between the acoustic boundary and fluidic boundary. The thickness of PDMS mold needed to be minimized to decrease the heating by the acoustic energy absorption of PDMS layer, which was successfully made using the spin-coating of PDMS and the UV tape transfer method. The acoustophoretic device with thin PDMS layer and optimized width of PDMS wall can be applied, for biotechnological applications such as the separation of blood cells and micro-particles.