Abstract: The expansion-temperature curves of a low alloy fine grain steel for -50℃ were measured by Gleeble 3800 thermal simulator at different t_8/5. The microstructure observation and hardness test of thermal simulated samples were carried out by optical microscope and microhardness tester. SH-CCT curves of the low alloy fine grain steel for -50℃ was obtained by analyzing the continuous cooling transformation（CCT） characteristics of base metal. The results show that the SH-CCT curves of HAZ of the low alloy fine grain steel for-50℃ can be divided into three transformation zones: martensitic transformation zone, bainite transformation zone and ferrite+pearlite transformation zone. When t_8/5is small, undercooled austenite is turned into bainite and martensite. With the increase of t_8/5, the morphology of bainite transforms from needle to lath, and the ratio of bainite increases gradually, even granular bainite appears. When t_8/5is bigger than 100 s, massive polygonal ferrite and black pellet pearlite appear. The hardness of thermal simulated HAZ sample decreases with the increase of t_8/5, while the impact absorbed energy increases first and then decreases.