Abstract: On the basis of the equilibrium conditions between gravity force and surface tension exerted on liquid metal, a static model is established to describe the surface shape of the deposited metal, the foundation of the model is the Young-Laplace Equation, which describes the pressure difference between the two sides of a curved liquid surface. Two dimensional hypothesis is adopted to simplify the calculation. The contact angle θ and the volume of filler metal on unit bead length are taken as two boundary conditions, so that the surface shape of the molten metal on a plane can be solved by an iteration algorithm. Then a static equilibrium model of the mechanism of the undercut phenomenon in high speed bead on plate welding is established, in which the geometry of the molten pool is considered. A'double iteration algorithm' is applied to get the equilibrium condition for the mixture of the filler metal and the molten base metal. The critical undercut condition calculated by this model corresponds well with the experimental result. Finally, the model is applied in analyzing the factors that act upon the undercut tendency. The analysis shows that by improving the wettability of the material, decreasing the bead width and increasing the volume of filler metal on unit bead length, the undercut tendency can be decreased.