Abstract: The heat-treatment process of bainite isothermal quenching was studied on ultrahigh strength steel 40CrNi2Si2MoVA.By bainite short-time isothermal quenching,the steel got a triphase structure of martensite,bainite and a little austenite.The effect of isothermal quenching time on the microstructure and mechanical properties of the material was also investigated.And the best parameters of hold time was defined by studying the mechanical properties and analyzing the metallographic structure of the steel.It is proved that the steel with fine strength-toughness got majority of martensite,20 percent bainite and a little austenite microstucture with 100-150 s isothermal time.