Abstract: In order to address the issue of sidewall non-fusion during narrow gap welding of TA31 titanium alloy, the wetting mechanism of the sidewall induced by magnetically controlled arc swing is investigated. The successful completion of welding a 42 mm-thick TA31 titanium alloy plate is achieved, followed by microstructure and mechanical properties analysis of the welded joint. The results demonstrate that magnetic-controlled arc swing enhances the wettability of liquid melt towards the sidewall, resulting in the formation of concave groove.This improvement in wettability primarily arises from alterations in temperature gra-dient between the molten pool and sidewall due to arc swing, as well as utilizing arc pressure and shear force to drive molten pool towards the sidewall. Microstructure analysis reveals the equiaxed β grains at the bottom layer is due to low heat input, and that the middle and cap layers exhibit large columnar β grains due to the competitive growth and epitaxial growth mechanisms in the cause of multi-layer welding. The α microstructures inside the β grains exhibit a typical basket weave morphology where the α plates overlap. Influenced by thermal cycle, the α phase coarsens significantly in the bottom layer. The average impact absorption energy of the weld zone is 55 J, approximately 74% of that of the base metal, and the heat affected zone demonstrates an average impact absorption energy value of 62 J, around 84% of that of the base metal. The magnetic control narrow gap welding technology can effectively resolve issues related to sidewall non-fusion and obtain TA31 titanium alloy joint with good microstructure and comprehensive mechanical property.