Abstract: The relationship between inclusions and the microstructure of the simulated weld-heat-affected zone (HAZ) in a microalloyed steel as well as low-temperature impact toughness was investigated. The results show that inclusions in the steel are mainly Ti₂O₃-Al₂O₃-MnS, these complex inclusions are well distributed and their size is less than 3 m. The microstrueture of HAZ consists of aeicalar ferrite, intergranular ferrite and small amount of lath bainite while the cooling time daring phase formation is short (T_8/5=40 s), the prior austenite grain size is about 50 μm and the samples have good impact toughness under this welding thermal simulation condition. The size of prior austenite grains increases with the increase of transformation cooling time (T_8/5=60 and 80 s), the increase of transformation temperature and the broadening of the transformation region make the promotion effect on polygonal ferrite induced by inclusions in the vicinity of the grain boundaries become more obviously, the intergranular ferrite grows up rapidly, the refinement effect of intragranular acicular ferrite is lowered to some extent, and the low-temperature impact toughness is decreased.