Abstract: To investigate the effect of magnesium on the properties of FH420 offshore engineering steel, two groups of experimental FH420 steels, with and without magnesium treatment, were smelted in a 25 kg vacuum induction furnace, followed by the application of identical temperature-controlled rolling and cooling processes. Thermodynamic calculations on the evolution of inclusions were performed via FactSage software, while the composition, size and number density of inclusions were statistically characterized using a scanning electron microscope, an energy dispersive spectrometer and an automatic inclusion analysis system. Meanwhile, mechanical properties were evaluated through room-temperature tensile tests and Charpy impact tests at -60 ℃. In addition, electron backscatter diffraction technology was adopted to analyze the matrix microstructure, effective grain size, as well as the micro-strain and crack propagation path beneath the impact fracture surface by means of IPF, KAM and GOS analysis. The results show that after the addition of 0.001 2% magnesium, the main oxide inclusions in the steel are transformed from Ti-Al-O system to Ti-Al-Mg-O system, with the inclusions significantly refined, the average size is reduced from 2.47 μm to 1.82 μm, and the number density of inclusions is increased from 54.01 mm^-2 to 71.85 mm^-2. After magnesium treatment, the microstructure of the steel is converted from coarse polygonal ferrite to fine interwoven acicular ferrite, and the effective grain size is decreased from 6.31 μm to 4.93 μm. Magnesium treatment exerts a slight influence on the yield strength, but the impact absorption energy at -60 ℃ is increased from 173 J to 234 J, with both the crack initiation energy and propagation energy improved remarkably. These results indicate that magnesium treatment can effectively induce the formation of acicular ferrite through modifying and refining inclusions in the steel, thereby refining the effective fracture unit of the steel. On the one hand, it enhances the capacity of the matrix to participate in plastic deformation; on the other hand, it forces the cleavage cracks to change directions frequently during propagation, thus consuming more fracture energy and improving the impact toughness of the steel. The effect of magnesium treatment on the properties of FH420 steel is systematically studied herein, which provides guidance for the research and development of new-generation products and the optimization of industrial processes for high-strength low-alloy offshore engineering steels serving in extremely cold environments.