Abstract: HG785D high strength steel was welded by metal active gas arc welding(80% Ar+20% CO₂), and theproperties of heat-affected zone for HG785 D high strength steel were studied. The analysis of the mechanical properties and microstructures of weld seam and heat-affected zoneshow that HAZ has lower impact property and high hardness. The microstructures in HAZ aremartensite and the upper bainitetransformed from sorbite in the base material. The microstructures inweld are ferrite and granular bainite because of many alloying elements added into weld by filler metal. The weld metal has good toughness. In HAZ, hardenability is high because of the presence of hard and brittle phases of martensite and upper bainite.