Abstract: EH36 steel thick plate is widely used in shipbuilding industry.In this paper,double-sided arc welding(DSAW) vertical welding of EH36 steel is carried out by robots,which exhibits distinctive merits and broad prospective in application.Backing weld is the most significant procedure in the multi-layer and multi-pass welding,which exhibits important impact on weld joint.Microstructure and performance of back welding joint were studied.The results indicated that proeutectoid ferrite(PF) and acicular ferrite(AF) were the main structure in the weld joint after DSAW.The structure in the back weld was refined after reheating effect in the subsequent multi-layer and multi-pass welding.Microstructure at coarse-grained zone of backing weld joint was bainite and maetensite lath.The hardness of back weld was slightly lower than that of the second layer bead.Joint tensile strength was higher than that of base metal,and the characteristic of fracture appearance was dimple.