Full penetration of mid-thickness metal with one pass welding can be obtained by K-TIG welding process. Single-sided welding and double-sided forming also can be performed, with high welding efficiency and good bead forming quality. Q345 R steel is often used as pressure vessel metal, and has to be welded through multi-layer and multi-pass welding by a traditional welding process, which is low efficiency and high cost. In order to expand the application range of K-TIG welding, external longitudinal magnetic field is combined to K-TIG welding and the microstructure and mechanical properties of Q345 R steel welded joints were studied. By changing the parameters of electromagnetic field, the influence of magnetic field on weld formation, joint microstructure and mechanical properties were studied. This study can lay a theoretical foundation for the optimization of K-TIG welding process.