Thin-film composite polyamide (TFC-PA) membranes prepared via interfacial polymerization (IP) are widely on reported in water treatment applications, but the inefficient mixing of reactive monomers in the traditional IP process may cause the diffusion-limited growth of PA layer and the incomplete IP reaction, resulting in limited control over the morphology and microstructure of PA layer, and thus the membrane performance. Various strategies to address above issues have been explored via different chemical modifications. In this work, a "green" ultrasound-assisted interfacial polymerization approach is employed for the first time to fabricate TFC membranes for forward osmosis and nanofiltration applications. Ultrasound in IP process enlarges the mixing area of reactive monomers, facilitates the mass transport of the amine monomer, therefore contributing to an efficient monomer mixing and the resultant higher IP reaction degree. Additionally, the disrupted PA chain packing, more penetrated amine monomers and generated nanovoids contribute to a relative loose PA layer. Effects of ultrasound power on the microstructure (crosslinking degree and free volume) and morphology (roughness and thickness) of the resultant TFC membranes are also investigated systematically. In comparison with the control membrane, TFC membranes formed via ultrasound-assisted IP exhibit much superior separation performance.