Abstract: The real welded joints and the method of weld thermal simulation have been employed in this paper to study the effects of chemical composition of domestic HSLA steels and cool lime t_s/s on ihe hardness of welding HAZ. On the basis of a great amount of data through measurement,the formulas for calculating the hardness of welding HAZ and thermal simulated HAZ were respectively established. The practical measurement results showed that the for mulas established for calcu-laling the hardness of real HAZ and thermal simulated HAZ are relialble.