To study the effect of Mg on super ferritic stainless steel, the content of Mg in S44660 super ferritic stainless steel was adjusted, and four kinds of test steels without Mg and with Mg additions of 0.0002%, 0.0004% and 0.0010% (mass fraction) were prepared. The effects of Mg on the cast microstructure and mechanical properties of S44660 super ferritic stainless steel were studied. The results show that the average grain size of the steel decreases from 1.14 mm to about 0.83 mm after 0.0002% Mg is added, and with the further increase in Mg content to 0.0004% and 0.0010%, the average grain size decreases to about 0.62 and 0.59 mm. It is confirmed that Mg can refine the grain of S44660 steel. Typical inclusion type of the S44660 steel is Ti-O-N composite inclusion, while it changes into Ti-O-Al-Mg-N composite inclusion after adding Mg, and the inclusion content and size decrease at the same time. The yield strength and tensile strength of the steel increase after adding Mg. Therefore, Mg can improve the impact absorption energy and hardness of S44660 super ferritic stainless steel.