In work, the modelling of a three-dimensional temperature field during surface modification of AlZn5.5MgCu aluminum alloy using GTAW (Gas Tungsten Arc Welding) technology is presented. GTAW is widely used for making welded joints using additional material (filler material). In the analyzed process, no additional material was used, and the effect of heat treatment was obtained by directly applying the electric arc to the surface of the material. The calculations were performed using Finite Element Method. The Goldak's double ellipsoidal heat source model has been used in modelling. The thermal-mechanical properties of the material were assumed to depend on the temperature. The Workbench, DesignModeler, Mechanical, Fluent and CFD-Post modules of the ANSYS program were used for numerical simulations. In the description of the geometry of element heat treated, cube type elements were used, with density of grid in the heat affected zone. The temperature distributions in cross-sections of heated element as well as welding thermal cycles at selected points were analyzed. The results of numerical simulations were verified experimentally. Comparison of calculated and obtained in the experiment the fusion lines showed satisfactory compatibility.