Abstract: The effect of heat input on the structure and mechanical properties of the welded joint in a new generation 800 MPa grade structural steel(RPC steel) has heen investigated. The results show that using the ultra low carbon bainitic welding wire to weld the 800 MPa grade RPC steel, the welded joints with the combination of high strength and good toughness could be achieved. With the increase of the heat input, the width of the dendrite increased. The primary ferrite and side-plate ferrite are almost removed completely in the secondary structure of the weld metal, and the lath bainite,acicular ferrite (AF) and granular bainite are the main micostructure components. When the heat input is increased, the tensile strength decreases gradually, but the low temperature impact toughness initially increases and then decreases. The peak of low temperature impact toughness at heat input of 20 kJ/cm is related to the fine and dense of AF obtained in the weld metal.