Arc brazing technology for joining has been recently developed. Because of its advantages, it has been used more and more frequently in the automobile industry, and this technology is widely used in practical joining applications. Many welding researchers have paid more and more attentions in this field. Based on the experimental analysis of CuSi3 filler metal spreading, wetting behavior on galvanized steel sheet and its interfacial microstructure in tungsten inert gas (TIG) arc brazing process, the governing equations and boundary conditions of filler metal droplets under the actions of gravitational force, surface tension and arc pressure force is conducted, then they have been transformed into finite volume equations, The flow behavior at different arcing time in filler metal droplet was calculated by PHOENICS code using SIMPLEST algorithm. It can be concluded that the flow adjacent to the surface controls the final droplet profile, and the fluid flow does great contribution to the whisker-like intermetallic compound fragmentation behavior due to its stirring force at the interface. The simulation is useful to interpret the free surface evolving behavior and the final profile of filler metal droplet in arc brazing, it also provide a new idea about the interface growth behavior study and offer a practical guide in production.