Abstract: Using a wetting balance technique, pure Sn was used to wet iron substrate assisted by ultrasonic treatment. By testing the wetting force curve of the tin-iron system, it indicates the wet process of the brazing filler metal to the base metal, the combination of the propagation characteristics of the ultrasonic wave and the micro-morphology of the base metal after wetting. The wetting behavior and interfacial reaction mechanism of the solder droplets on the surface of the iron were studied with the ultrasonic effects. The results reveal that ultrasonic assist in the wetting force test of the Sn-Quartz system can increase the wetting force of the brazing filler metal to the base metal and in the wetting force test of the reaction system can accelerate the wetting time and increase the wetting force of the brazing filler metal to the base metal. With the increase of ultrasonic time and ultrasonic power wetting time of solder is reduced, the wetting force is increased and solid/liquid interface reaction is intensified, interface generated more and more thick more and more dense intermetallic compound, the base metal surface relative to the original surface become rough, thus easier to wetting process, and the reduction of the surface tension of the base metal, so that the wetting force is increased.