The reliability of the joint is mainly determined by the morphology of interfacial IMC and microstructure in joint. In the ultrasonic-assisted soldering process, the behavior of the base metal being dissolved by the liquid solder affects the microstructure of the joint. In this paper, the dissolution behavior of Ni in molten Sn solder with and without applied ultrasonic vibration (USV) was investigated through the immersion experiment. In order to reveal the mechanism of accelerated dissolution by USV, the ultrasonic pressure distribution in molten solder were simulated by finite element analysis and the microstructures after dissolution were observed. The results showed the dissolution amount of Ni in Sn after 10 s of USV was equivalent to a holding time of 5 min without USV. Without USV, the interfacial IMC gradually thicken as the holding time increases, which hinders the diffusion of Ni atoms into the Sn solder. With USV, ultrasonic cavitation caused the infinite dissolution of Ni into tin solder. Furthermore, the dissolved Ni atom at the interface was brought rapidly to the molten solder by the ultrasonic streaming, which cause a large number of elongated rod-shaped Ni3Sn4 precipitated during the subsequent cooling process.