Abstract: With relevant thermal simulation tests, continuous cooling of four high-strength low alloy steels at different cooling rates were conducted. Also conducted with the said steels were thermal circulation of tempering at different temperatures. Analyzed with the said tests were such aspects concerning the steels as separation pattern and resolution of M-A （martensitic-austenitic） structure,percentage of the M-A structure and effects by austenitic grain size and chemical element contents on the hardness and impact toughness of the heat-affected zone （HAZ）. It is concluded that since lump M-A is mainly responsible for notch toughness of the steels,it is possible to reduce or resolve the lump M-A structure to improve the HAZ notch toughness by means of proper control of continuous cooling time and post-cooling tempering.