Abstract: This paper explores laser-MAG hybrid welding of 18 mm-thick AH36 marine high-strength steel. Experimental investigations on weld formation are conducted with two groove-parameter variables, i.e., the root width of U-groove and the included angle of V-groove. Combined with macro-morphology observation, high-speed photography for droplet acquisition and weld cross-section analysis, the influence of groove configuration on arc behavior, droplet transfer, molten‑pool flow and welding defects is clarified. The results indicate that for U-grooves, the restraint of groove sidewalls on the arc and molten pool intensifies with decreasing root width. Small root widths readily induce welding defects including humping, spatter and incomplete side‑wall fusion. Increasing the root width can mitigate side‑wall constraint and optimize arc and droplet transfer behaviors. For V-grooves, the groove volume rises with the increase of groove angle, leading to prominent heat accumulation. Excessively large groove angles aggravate droplet deviation caused by wall contact and increase the risk of internal micro-defects in welds. Comparative analysis of the two groove configurations demonstrates that the U-groove with a root width of 6 mm realizes symmetric and stable arc combustion as well as steady fine-droplet transfer, and achieves the optimal comprehensive forming quality for both front and back weld surfaces.