Abstract: The welding of 8-mm thick Q235 low-carbon steel plates by keyhole tungsten inter gas welding (K-TIG), a deep penetration argon arc welding technique with tungsten electrode, is associated with many problems, including an unstable welding process and a small welding current window. To solve these prominent problems, the method of adding shielding flux on the back of the welding workpieces was proposed for the first time in this paper. This method can improve the stability of the welding process. The butt welding method was used to achieve the result of single-sided welding and double-sided forming without adding welding wire or prefabricating groove during the welding process. The results show that direct current (DC) in the range of 430–480 A is successfully used to weld the 8-mm thick Q235 low-carbon steel. The welding current window is expanded to 50 A, and the welding process stability is significantly improved. After expanding the welding current window, the microstructures and properties of welded joints obtained under different welding currents were systematically studied. The results show that the distribution of microstructures and the mechanical properties of the welded joints under different welding currents present the same states. The microstructures of the weld zone are composed of ferrite + pearlite + widmanstatten structure; the microstructures of the fusion zone are composed of Widmanstatten structure; the structures of the heat-affected zones are composed of ferrite + a small amount of pearlite. In addition, with the increase in the welding current, the fusion width of the back of the workpiece increased slightly. In the welding joint, the hardness value of the fusion zone is the highest, followed by the weld zone, and the heat-affected zone. The base material has the least hardness, and the final tensile fracture position of the welded joint is in the heat affected zone.