Abstract: Objective The aim is to enhance the process stability of high-current D-Arc buried-arc welding for large-sized steel structures in medium-to-thick plates, and to provide analytical basis for the microstructural and mechanical property evolution of welded joints under high heat input conditions. Methods This study investigates the relationship between different welding currents and the droplet transfer mode, droplet transfer size and frequency of D-Arc buried-arc welding through process trials. It also analyzes the influence of heat input on microstructure and mechanical properties. Results The results indicate that, as the welding current increases, the tendency for non-axial repulsive droplet transfer grows stronger, shifting gradually from oscillatory tansfer to rotational transfer. Droplet size progressively increases, while the droplet transfer frequency exhibits a trend of first increasing and then decreasing due to the influence of metal vapor reaction forces. The joint microstructure develops coarse grains, and upper bainite inclusions appear in the heat affected zone, leading to a deterioration in the overall mechanical properties of the joints. Conclusion While ensuring full penetration of the plates, welding current should be strictly controlled to preserve overall mechanical properties of the joints.