Abstract: The plutonium (Pu) isotopic composition in mixed uranium-plutonium oxide (MOX) fuel significantly influences the core parameters of large sodium-cooled fast reactors (SFRs). Multiple representative MOX core configurations derived from spent fuel assemblies were selected as feedstock. The fast reactor core neutronics optimization code CoreEasy was employed to adjust fuel enrichment, followed by detailed analysis using the 3D hexagonal nodal code NAS to evaluate key parameters of SFR cores with varying Pu isotopic ratios. These parameters include power distribution (axial/radial power profiles, maximum linear power density, and power peaking factors), reactivity effects (temperature coefficient, power coefficient, sodium void reactivity, burnup reactivity loss, and Doppler effect), and control rod worth. Computational results indicate that the isotopic composition of recycled Pu in MOX fuel moderately impacts core behavior. Specifically, a reduction in the fissile isotope fraction (²³⁹Pu and ²⁴¹Pu) weakens negative feedback mechanisms, such as temperature, power, and Doppler effects, though the magnitude of this attenuation remains limited. Concurrently, sodium void reactivity exhibits a significant increase of 10.96% to 33.15%, driven by neutron spectrum hardening and reduced neutron leakage during sodium density perturbations. In contrast, power distribution profiles (both axial and radial) and control rod worth demonstrate negligible variations across all configurations, confirming their stability.From a safety perspective, higher fissile isotope content (²³⁹Pu and ²⁴¹Pu) enhances beneficial reactivity effects (excluding burnup reactivity), thereby improving inherent safety margins. Power distribution and control rod worth remain largely unaffected by isotopic variations, posing no additional safety risks. Overall, the influence of Pu isotopic ratios on core performance falls within acceptable operational limits, validating the feasibility of utilizing MOX fuels produced from diverse spent fuel precursors in SFRs through optimized core loading arrangements. However, to maximize safety and utilization efficiency, practical applications should prioritize MOX fuels fabricated from recycled Pu enriched in fissile isotopes (e.g., ²³⁹Pu and ²⁴¹Pu). This strategy optimizes negative feedback characteristics while mitigating sodium void reactivity risks, ensuring compliance with stringent safety criteria for maintaining a negative void coefficient. The study underscores the adaptability of fast reactors to heterogeneous Pu isotopic compositions, and provided critical thresholds for fissile isotope content are maintained.