In this study, the wetting behavior of Sn solder on Cu substrate under ultrasonic vibration using the sessile drop method was investigated. The distribution of the vibration field on the substrate surface was analyzed via COMSOL Multiphysics finite element analysis, while the spreading behavior and interfacial reaction products of intermetallic compounds (IMCs) induced by varying ultrasonic amplitudes were explored. ultrasonic vibration demonstrates a significant influence on both wettability and interfacial reaction products. The wetting mechanism under ultrasonic vibration is primarily attributed to physical driving forces generated by ultrasound-induced interfacial viscous boundary layers, along with chemical driving forces facilitated by ultrasonic-accelerated interfacial atomic diffusion and reactions.