The effect of undissolved ferrite amount on impact fatigue properties and failure mechanism were studied by using 42CrMo steel with subcritical quenching process The amount of undissolved ferrite were 0%, 10% and 15%, respectively. The experimental results show that the existence of undissolved ferrite can not only change the microstructure, but also increase the impact fatigue life The impact fatigue life elongates with increasing of amounts of undissolved ferrite The grain can be fined by using subcritical quenching process and the area of phase boundaries can also be greatly increased because of undissolved spheroidal carbide. The martensite and carbide form can also be changed by using subcritical quenching process The stress relaxation due to the moving of dislocations inside the ferrite and the promotion of strength due to occurring of plastic deformation and the enwinded dislocations are main reasons of improving the impact fatigue life. The impact fatigue life elongates with the increase of amounts of undissolved ferrite before the amount of undissolved ferrite reaches 10%. Under the experiment conditions, when the amount of undissolved ferrite is 10%, the impact fatigue life will be the longest.