The localized corrosion behavior of EH690, a 690 MPa grade low alloy steel in an artificial chloride containing acidic seawater was assessed via immersion test, optical microscope, laser confocal microscope, and scanning electron microscope with energy dispersive spectroscope, in terms of the effect active inclusions on the localized corrosion of the steel. It is found that the inclusions in EH690 steel are similar in shape, mainly spherical or ellipsoidal, with a diameter of 1-4 μm. The types of inclusions in steel plates of different thickness are basically the same, which may be divided into three categories: (Mn,Ca)S-MgO·Al₂O₃, (Mn,Ca)S-MgO·Al₂O₃-TiN, and (Mn,Ca)S. The density of the corrosion-active inclusion is higher on the surface area of the thick plate. The dissolution of corrosion-active inclusions first occurs at the interface between the inclusion and the matrix, meanwhile causes corrosion of matrix. However, in case when immersion in the same medium for the same period, these inclusions exhibit different corrosion activities, which may be due to the different diffusion capabilities of aggressive particles in the pits and the micro-galvanic corrosion within the pits.