To reduce the construction cost and period of steel structure, high-efficiency welding methods applying high heat input have been widely employed. However, deterioration of the strength and toughness, especially low-temperature impact toughness, due to the coarse grains in the heat-affected zone (HAZ) of weld steels often comes along with high heat input welding. Thick offshore steel has been developed by Ti deoxidization for the use of high heat input welding. The inclusions of the steel were analysed by EPMA and SEM, and the major inclusions were identified to be Al O -(MgO) and Al O -CaO-(TiO ) types. The steel plates with different thickness over 40 mm were welded with heat input over 100 kJ/cm, and Charpy impact energies of the welding joints at −40 °C were over 75 J. The nano-sized TiN particles in HAZ were observed and were considered to retard the grain coarsening in the HAZ during high heat input welding.