Abstract: In the process of vertical narrow gap MAG (NG-MAG) welding, arc-swinging and time-independent fusion technology is the key to ensure good sidewall fusion. Through analyzing the change rule about velocity and direction of arc-swing, the dynamic heat source model of NG-MAG vertical welding was established by using the method of coordinate transformation based on the classical double ellipsoid heat source model. Temperature field simulation results show that the welding thermal curves are "multi-peak", weld surface shape is "building blocks" and cross section is "dumbbell", which are in consistent with the test result of NG-MAG welding. The "multi-peak" phenomenon is mainly caused by the arc heat source moves near and then away from the side wall, whiles the formation of "building blocks" and "dumbbell" are highly correlated to the periodic change of arc speed (heat input) and weld pool during arc-swing. The above-mentioned characteristics provide the technical support for further research on microstructure evolution of high strength low alloy (HSLA) steel in NG-MAG vertical welding process.