Abstract: The microstructure and properties evolution behavior in direct quenching and tempering process of a low-alloy high-strength steel for engineering machinery was studied to provide experimental basis for online production of offline quenched and tempered high-strength steel.Coarse granular bainite structure was obtained under air cooling after rolling and the impact toughness was lowered.All the microstructure obtained was lath martensite when it was fast cooled to below 210℃.The experimental steel exhibited good temper resistance,and the strength was still high enough after high temperature tempering.The comprehensive mechanical properties of the steel under direct quenching and tempering were better than those under offline quenching process.The result shows the advantage of online heat treatment technology under ultra fast cooling condition in the development of low consuming and high performance steel.