Abstract: The static and dynamic CCT curves of a low alloy low density steel were measured by using Gleeble 3500 thermal simulator, thermal expansion method and microstructurec hardness measurement. The results show that under the condition of low cooling rate, the structure is mainly ferrite, M/A island and pearlite. With the increase of cooling rate, the structure of static sample has no obvious change, and the content of M/A island gradually increases. The dynamic sample starts to appear bainite at the cooling rate of 20 ℃/s. The higher the cooling rate, the higher the content of bainite in dynamic sample, and the lower the content of M/A island, which indicates that plastic deformation promotes the formation of bainite.The shear reaction of martensite is hindered to inhibit the formation of martensite. At the same time, the plastic deformation induces the increase of ferrite nucleation points, which refines ferrite grains.