In recent years, the components tend to be large-scale and large-span. In order to improve the welding construction efficiency and reduce production costs, the high input welding methods, such as automatic gas electric vertical welding, submerged arc welding, electro slag welding, etc., have been widely used in manufacturing fields, like shipbuilding, buildings, bridges, petrochemical and marine structures, etc.. The domestic iron and steel enterprises and research institutes have cooperated successively to develop a number of heat input welding steel with heat input greater than 400 kJ/cm. However, at present, the welding materials which can be matched with these special steels are still dependent on import. In order to change this passive situation, a new type of flux cored wire has been independently developed in this research. The effects of heat input on the microstructure and impact toughness of the weld metal have been investigated through laboratory tests. The results demonstrate that under the condition of large heat input welding, a large number of fine inclusions are formed and distributed randomly in the weld metal. Substantial amount of interlocked acicular ferritic grains are found around the inclusions, contributing to the high impact toughness for the weld metal. With the increase of heat input value, the number of fine inclusions (smaller than 1 μm) decreases and the tendency of inclusion assembly and growth is found to accelerate. Simultaneously, the nucleation points of acicular ferrite decreased and the grain size of acicular ferrite increased slightly in the weld metal. The impact toughness was deteriorated mildly as well.