In the case of welding of T-joints in a special structure, the joining is realized through a total penetration of deck plates. For the deck plate’s thickness over 6 mm, high power MAG welding process should be applied. To help experimental optimizing of this welding technology, a 3D quasi-stationary numerical model was established to predict the penetration and weld form of the high power MAG welding on a thick plate. In the analysis a new volumetric heat source model was put forward which considers the heat directly from the arc und that from transferred droplets separately. Because the weld pool surface under the arc was strongly pressed, the droplet heat source in the model was located under the workpiece surface. The size of the droplet heat source model was determined on the base of physical principles and available experimental data. Using a commercial finite element software the weld form inclusive penetration under different welding parameters was then simulated. Through comparison with the experimental results the presented FE-model was verified.