Abstract: To address the insufficient deep penetration in conventional keyhole tungsten inert gas-metal inert gas (KTIG-MIG) hybrid welding and enhance the efficiency and quality of medium-thick plate welding, a novel KTIG-MIG hybrid welding method was developed, combining the deep penetration capability of KTIG with the high cladding rate of MIG. Hybrid welding tests investigated the effects of process parameters on coupled arc morphology and molten droplet transfer behavior, while a combined heat source model analyzed temperature field variations in butt joints. The results show that in a certain range of welding parameters, the increasing KTIG or MIG current expands the composite arc’s bottom width, with optimal coupling achieved at KTIG current of 350 A, MIG current of 220 A, and MIG arc voltage of 22 V for Q235B low-carbon steel plates with a thickness of 8 mm. The KTIG arc significantly alters MIG molten droplet transfer, and larger wire-to-electrode spacing shifts the molten droplet transfer mode from projected droplet to globular or short-circuit transfer, whereas higher KTIG current promotes spray transfer. Temperature field simulations of the joint reveal that wire-to-electrode spacing critically affects heat input and approximates an inverse proportionality with welding thermal efficiency.