Abstract: Samples were taken from different parts of the forged 42CrMoS steel bar with a diameter of 380 mm, and then were treated by water cooling quenching or air cooling quenching and high temperature tempering with the same process. The microstructure, morphology and quantity of sulfide inclusions and their effects on the transverse impact toughness after heat treatment were studied. The results show that there was no serious element segregation in the steel bar. The transverse impact absorption energy of different parts under water cooling quenching and air cooling quenching conditions was 21.6-35.6 J and 12.7-17.7 J, respectively. The morphology of sulfides in the steel bar had more significant influence on the transverse impact toughness than the quantity. The fine and uniformly distributed point sulfide had relatively little adverse effect on the transverse impact toughness. When every 10% area fraction bainite was introduced into the microstructure of the air cooling quenched and high temperature tempered samples, the transverse impact absorption energy decreased by about 6%, and the degree of the influence of tempered sorbite on the transverse impact toughness was about 2.5 times that of the bainite.