Abstract: To meet the reserach demand of weld formation in narrow gap MAG welding, the welding temperature field was numerically simulated by Ansys to study the affecting factors. Considering the characteristics of this welding method, rotating load of the heat source model was realized and three-dimensional temperature field numerical calculation was obtained. The numerical simulation results of weld shape and the experimental results of actual welding were in good agreement. In order to further improve the computational efficiency, a similar saddle ring equivalent heat source was proposed according to heat equivalent and characteristic of heat source distribution. The simulation results of sidewall penetration agreed well with the practical measurement, and the calculation speed was greatly improved. The results were helpful for the further research on penetration control and welding deformation.