A bionic airfoil based on a silent-fight long-eared owl wing was built by using root mean square. The flow structure around the owl wing and the related source were revealed. The unsteady flow showed that a leading-edge boundary layer was separated, and later a bubble was formed due to instability. Furthermore, a vortex detached from the bubble, drifted downstream, and immediately broke down later. A turbulent boundary layer was reattached on the upper surface. A similar evolution occurred on the lower surface, but the vortex drifted further downstream. The corresponding far-field directivities suggested dipole noise and the maximum intensity was at the trailing edge. It suggested that the scattering noise was dominant. In the spectrum, the noise was mainly concentrated in the mid frequency and the tonal characteristics also agreed with the flow structure. The noise of the bionic airfoil started to drop significantly around 1kHz, and the similar trend was also gained for the natural owl wing with decrease above 2kHz.