Abstract: To address the segregation issue in the radial 1/4R~1/2R region of 42CrMoA alloy structural steel hot-rolled bars, this paper systematically elucidates its formation mechanism and proposes improvement measures. These measures were validated through industrial production, ultimately establishing a clear control method for segregation in rolled materials. The results indicate that segregation in rolled materials is inherited from the CET zone (Columnar to Equiaxed Transition) segregation in the casting billet, primarily caused by the disordered growth of the CET zone hindering solute diffusion, combined with the cavity suction effect induced by electromagnetic stirring and billet reheating. By reducing the Mold Electromagnetic Stirring (M-EMS) current from 400 A to 200 A and adopting low superheat, the standard deviation of carbon content on the cross-section of rolled materials decreased from 0.024 to 0.018, the carbon range decreased from 0.073 to 0.05, and the segregation index decreased from 1.090 to 1.057, significantly improving compositional uniformity. Adjusting M-EMS intensity demonstrated a more pronounced effect on mitigating segregation. Ultimately, the optimal segregation control method for 42CrMoA rolled bars was determined as follows: M-EMS current of approximately 200 A and superheat of around 30 ℃.