In order to investigate the effect of cerium on the carbon segregation in steel, a thermodynamic calculation mold of equilibrium partition coefficient in multi-component alloy is established. The equilibrium partition coefficient of C in Fe-C-Ce system with different cerium content is calculated with Equilib moudle of FactSage. Results show that the effect of cerium on phase transformation is remarkable during solidification. The addition of cerium facilitates austenite transformation and enlarges the L-γ coexist zone. Cerium increases the average equilibrium partition coefficient which means it can mitigate the segregation of C in steel. The optimum addition content of cerium to improve the segregation of C in steel are associated with carbon content. For Fe-0.1%C-x%Ce system, the optimum addition content of cerium ranges from 0.047% to 0.057%. And for Fe-0.25%C-x%Ce system, the optimum addition content of cerium ranges from 0.03% to 0.04%.