Liquid metal fast reactors (LMFRs) are prominent among the fourth-generation advanced nuclear energy systems due to their outstanding advantages in fuel breeding, nuclear waste transmutation, and inherent safety. The breeding of nuclear fuel is a key issue in the advanced nuclear energy system, which is not only of fundamental scientific significance but also has important application prospects and is central to the sustainable development of nuclear energy. Fuel breeding refers to the generation of more fissile nuclides than are consumed in a reactor, involving the composition of uranium–plutonium fuel and its fission process. Based on a lead-cooled fast reactor, the fast neutron reactor physics analysis code SARAX-CGN was used to simulate the uranium–plutonium composition of mixed oxide (MOX) fuel, and the impact on parameters such as burn-up reactivity, breeding ratio, void effect, doppler effect, neutron kinetics, and core neutron spectrum was analyzed. The results indicate that as the mass or quality factor of plutonium in the fuel increases, the average breeding ratio over the life tends to decrease, and the maximum reactivity oscillation value over the life cycle first decreases and then increases. Under the same fuel loading, the MOX fuel core requires a relatively lower mass of fissile nuclides, and the doppler effect is more pronounced. The effective delayed neutron fraction of the MOX fuel core is significantly smaller than that of the UO2 core, which is unfavorable for accident analysis and needs to be considered in control rod design. The MOX fuel core significantly reduces the overall void effect in the core. The spectrum of the MOX fuel core is relatively softer. Research on the characteristics of uranium–plutonium fuel from the perspective of breeding and analysis of its impact on neutronic parameters provides a theoretical basis for the design of fast reactor cores.