The present article covers the solution of the thermal task under MIG/MAG welding of ends of a joint that are subject to overheating due to reflection of heat flow from the edge. For the calculations of the reflected heat flow from the edge we introduced simulated heat sources in the calculation model. Obtained model allows us adequately estimate temperature distribution in quasi-steady state as well as in the area of thermal pattern change under the reflection of heat flow from the edges of weld part