Abstract: According to the problem that the mechanical properties of 30CrMnSiA steel hook after isothermal quenching was not up to standard. The effect of isothermal temperature on the microstructure and mechanical properties of the material were analyzed through the process test. The optimum process parameters of isothermal quenching were determined. The results showed that the composite microstructure of bainite and martensite could be obtained by isothermal quenching of 30CrMnSiA steel. With the increase of isothermal temperature, the upper bainite content gradually increased, and the hardness and strength values decreased.