Abstract: Based on the arc deflection of the new type swing arc narrow gap melting electrode gas shield (GMA) welding, the arc heat source staying at the sidewall was decomposed into arc component heat source and sidewall heat source, and the heat source model considering the sidewall heat source was established. The finite element model of the new type swing arc narrow gap GMA welding was established by ANSYS software. The cross-section fusion line profile and thermal cycle curve of welded joint with and without considering sidewall heat source were simulated and verified by experiments. The temperature field in the swing arc narrow gap GMA welding with considering sidewall heat source was studied by the finite element simulation method, and was compared with that without considering sidewall heat source. The results show that the difference between the depth of the highest point of the fusion line on the left and right sides of the weld cross section by simulation with considering sidewall heat source and the test result was smaller than that without considering sidewall heat source. The variation trend of the thermal cycle curves at different test points of the welded joint by simulation with and without considering sidewall heat source was basically consistent with the test results, and the maximum relative errors of peak temperatures were 1.8% and 3.4%, respectively. The heat source model considering the sidewall heat source could more accurately describe the heat source distribution characteristics in the swing arc narrow gap GMA welding. The bottom weld penetration depth by simulation with considering sidewall heat source was smaller than that without considering the sidewall heat source, and the side weld penetration depth was larger, but the size change was less than 0.1 mm. The formation law of the molten pool before the temperature field stabilization was the same as that without considering the sidewall heat source, but the molten pool size on the upper surface of the joint was smaller; after the temperature field was stabilized, the molten pool size on the upper surface of the joint was the same.