Liquids and reagents mixing at the microscales are of great significance to various lab-on-chip applications such as drug delivery and microreactor. This work demonstrated mixing enhancement based on microstreaming induced by ultrasonically activated attached bubbles in a straight microchannel. The bubbles were experimentally trapped in a microchannel fabricated with hydrophobic surfaces, and they demonstrated good stability to flow rate due to the lower surface energy of the system. The effect of the applied voltage and flow rate on the mixing performance was numerically and experimentally studied. Both results show that reducing the flow rate or increasing the voltage would improve the mixing efficiency. Furthermore, the proof-of-concept was successfully tested by utilizing it as a microreactor for sodium hydroxide and phenolphthalein at a throughput of 4 μL/min. The proposed chip can achieve diverse mixing processes based on the trapped bubble state than other microfluidic mixers. We believe the proposed design will be valuable for developing bubble-based biochemical and biological applications.