The article is devoted to development of a technique, which allows calculation of the welding mode of treatment of V-shaped groove for formation of a root seam. The technology of the multipass welding of MIG/MAG (arc welding in a mixture of the shielding gases) of the thick-walled structures is considered. An analysis of the problem statement demonstrated that the main problem is calculation of the penetration technique into the base metal for the V-shaped groove welding. Therefore, the authors proposed a proprietary technique for calculation of penetration using a finite element modeling (FEM). As a result, a heat input regressive model was developed for a particular form of the edge grooving, which is parametrized with respect to the gap and welding speed. For obtaining of an adequate penetration into the base metal, it is suggested to complicate the source model structure of the welding heat and to use the system of two heat sources of a "double ellipsoid" type. The analysis of penetration was done due to FEM solution of 3D heat equation, taking into account the melting and crystallization processes. The 3D heat equation task for the solid multiphase environment was solved. Further, an algorithm for calculation of the welding stability and feasibility was developed. The n2 dimensionless similarity criterion was used. The proposed method allows us to determine the stable welding conditions for the given range of the welding conditions variation by n2 criteria calculation without numerous welding experiments. Calculations of the sustainable modes of the root pass with 2,0...3,5 mm gap and 2,0...9,0 mm/s welding speed were performed.