Abstract: The 12 mm thick Q235 steel eutectic pool backing welding is realized by using tandem welding torches matched with different DC pulse frequencies. High-speed image (HSI) and real-time electrical signals are used to monitor the welding process in real time. The effects of welding speed on weld formation, arc morphology and welding process stability are studied. The results show that the welding speed mainly affects the weld forming and welding process stability, but has little influence on the arc morphology. With the increase of welding speed, the arc stability increases first and then decreases. When the welding speed reaches 40 mm/s, the best back free forming is achieved. At different welding speeds, the leading wire arc maintains a high stiffness and hardly deviates. The trailing wire arc forms a regular swing of 9° − 15° in a single pulse cycle, which promotes the metal in the middle of the molten pool to move to the side. The molten flow reduces the metal accumulation at the rear of the molten pool, thereby eliminating hump and ridge defects on the weld surface and improving the overall finish. The welding stability of the leading wire is generally higher than that of the trailing wire. When the welding speed is 40 mm/s, the stability of the arcs is the best.