The large primary carbides in H13 steel significantly affect its service life. In this study, pre-dispersion technology is used to disperse CeO2 nanoparticles, which are then introduced into H13 steel. The effects of CeO2 nanoparticles on the inclusions, primary carbides, microstructures, and properties of H13 steel are investigated. Ce-containing inclusions with size of 0.5–2 μm are generated and distributed uniformly in H13 steel owing to the reduction of CeO2 by Al and C. Both the dendrite structure and primary carbides in H13 steel are refined by CeO2 nanoparticles. The secondary dendrite arm spacing decreases from 13.51 ± 2.34 to 6.90 ± 1.23 μm. The size and number of primary carbides decrease from 1 to 5 to 1–3 μm, and from 1125 to 725/mm2, respectively. This is attributable to the disregistry of Ce-containing inclusions and primary carbides/austenite. The segregation of alloy elements in H13 steel is calculated using the Clyne–Kurz model. The properties of H13 steel are improved via the addition of CeO2; in particular, its tensile strength, total elongation rate, and hardness are 726.38 MPa, 23.43%, and 243.49, respectively, which corresponds to increases by approximately 5.58%, 8.27%, and 10.11%, respectively. The results can facilitate improvements to the performance of H13 steel and the development of rare earth steel.