The FH40 ship plates containing different Mg, Zr contents are refined with vacuum induction furnace, and the relationship between magnesium and zirconium addition and the decomposition of austenite during continuous cooling rate from 0.1 ℃/s to 30 ℃/s is systematically investigated by a Gleeble-3800 thermal simulator. Continuous cooled microstructure is observed by means of optical microscope. Dilatometric experiments and metallographic observations show that, Mg, Zr and Mg-Zr addition can affect the starting temperatures of the γ-α phase transformation and the pearlite transformation. Mg and Mg-Zr addition suppresses the proeutectoid ferrite transformation and promotes the bainite transformation. The matrix can be dominated as bainite at cooling rate from 5 ℃/s to 30 ℃/s for the Mg-containing steels. It is obviously shown that the addition of zirconium has a significant effect in promoting the nucleation of ferrites. However, the bainite transformation is not well developed, but an evident suppression of the transformation is emerged for the single Zr addition steel. The above studies can provide theoretical foundations for the application of magnesium and zirconium in steel.