Abstract: Elaborated in the paper are the development background and the R & D activities of the MK65HGX-Ⅲ welding wire for making of the SAWL pipe for deep ocean pipeline service. 115, 250 tons of X65 heavy-wall linepipes have been made by the welding process employing the said welding wire and the optimized H08DG welding wire together with the SJ102G high-basicity sintered agent. The linepipes are in compliance with applicable standard in terms of low temperature toughness,rupture toughness and weld joint transversal tensile strength,and also have stable performance,and rather large allowance. Relevant operation practice demonstrates that as a new type hi-performance welding wire for SAW process,the MK65HGX-Ⅲ welding wire is capable of making steel pipes with high strength,high toughness and high plasticity,being suitable for welding operation with a large linear energy(approximately 65 kJ/cm),and worth spreading for welding pipes in higher steel grades(e.g.,X80),and pipes as required for proper H2S corrosion resistance.