Abstract: Multi-layer and multi-pass welding is a common welding process for large-diameter thick-walled applications. As a direct factor affecting the number of weld layers and passes, the groove form not only determines production costs and efficiency but also further influences the arrangement of weld seams and the operability of the welding torch during the welding process. In this paper, experiments on 20°, 15°, and 9° U-groove were conducted based on TIG root welding and MAG filling and capping welding processes. The arc stability, weld seam arrangement, and welding torch operability under different groove forms were analyzed. The results indicate that the groove size has virtually no impact on arc stability. The arc can adequately heat and fill both sides of the groove while ensuring a relatively smaller fill volume under the 15° U-groove. Therefore, 15° U-groove offers good operability and high welding efficiency for root welding.