Abstract: Fine microstructure in the fusion zone of HQ130 steel (σ_b≥1300MPa) is studied by means of metalloscope,SEM,TEM and electronic diffractometer.Experimental results indicate that although partially-melted of the fusion zone experienced δ→γ→α(ML) transform,ordered state of link crystal between base metal and the weld metal has not been damaged in this zone. Under of the condition of CO₂ and Ar+CO₂ welding,width of the fusion zone of HQ130 steel is only 20-60 μm. The fusion zone of HQ130 steel consists of different microstructures, ML,AF,PF,Bg,etc.,and has obvious unevenness. To prevent toughness decreasing resulted from crystal coarsing in the partially-melted zone,controlling weld heat input (E≤20kJ/cm)is very important during welding.