The thermal conductivity of nuclear fuel elements is directly related to the safety and economy of reactors, as traditional UO₂ fuel can lead to excessively high central temperatures due to its low thermal conductivity. Introducing high thermal conductivity metal inserts between UO₂ fuel pellets, using a pellet-insert stacked structure, can significantly optimize the thermal performance. This paper also establishes a novel equivalent thermal conductivity model to investigate the equivalent thermal conductivity of this structure. Based on Fourier’s law of heat conduction, the model quantifies the heat conductivity enhancement effect of the pellet-insert stacking structure using a homogenization assumption and a one-dimensional heat conduction equation. Numerical simulations of UO₂-Mo stacked fuel elements are conducted using ABAQUS finite element software, analyzing the temperature field distribution and equivalent thermal conductivity characteristics for different insert thicknesses. The results show that a 0.1 mm thick Mo insert can reduce the peak temperature of the pellet by 10% (from 2129.57 K to 1912.85 K). A radial distribution model of the equivalent thermal conductivity enhancement factor, fitted using a cubic polynomial, provides high-precision input parameters for a 1.5-D fuel performance analysis program, thus avoiding the errors of traditional equivalent assumptions. The study further reveals the complex relationship between insert thickness and thermal conductivity gain, offering theoretical support for high-power reactor fuel design. Future work will involve verifying the model with irradiation experiments, exploring the optimization potential of multi-material inserts and non-uniform layouts, and extending the analysis to transient conditions and multi-physics field coupling. This model provides a quantitative tool for the thermal design and safety assessment of nuclear fuel elements, with significant engineering implications for improving reactor efficiency and reliability.