In order to study the influence of different refining processes on sulfides in ring gear steel 42CrMoS4 steel， starting from the morphology and distribution of sulfides in the steel， two different alkalinity slag and calcium content processes were designed in the refining process， and four heats were tested for production. The characteristics of composite sulfides with different double-layer structures and the formation mechanism of sulfides in cast billet and rolled steel were compared and analyzed. The results show that after LF makes high alkalinity slag （

R

=6.5～7.2） into RH without calcium treatment， the composite sulfide at 1/4 thickness of the casting billet is in a block morphology as a whole， and the oxide inside the core is also a regular block， the internal oxide of the rolled steel is mainly CaO， and the peripheral sulfide is mainly （Ca， Mn）S with a high CaS ratio， which is basically not deformed， and the grade of class A fine inclusions is 2.0～3.0. After LF makes weak alkalinity slag （

R

=3.6～4.2） into RH secondary calcium treatment， the Ca content is increased from about 0.001 4% to 0.002 5%～0.003 2%， the composite sulfide at 1/4 thickness of the casting billet becomes an ellipsoidal morphology as a whole， the core internal oxide is also regular spherical， the internal oxide of the rolled product is calcium-magnesium-aluminate with a low CaO ratio， the peripheral sulfide is mainly （Ca， Mn）S with a low CaS ratio， and the sulfide is spindle-shaped， which has a good deformation ability ，the level of class A fine inclusions is controlled within 2.0.