To address the lack of quantitative comparative studies on the coupled effects of geometric parameters on phase-change heat transfer performance in eccentric elliptical cross-sectional radial gravity heat pipes, this work systematically investigates the effects of eccentricity and elliptical cross-sectional aspect ratio on the internal temperature field distribution, flow characteristics, and total thermal resistance based on a three-dimensional two-phase flow numerical model. The results indicate that the total thermal resistance exhibits a nonlinear trend, first decreasing and then increasing with increasing eccentricity. For the investigated EE-RHP geometry with an outer tube of 60 × 40 mm and an inner tube diameter of 25 mm, an eccentricity of 10 mm yields lower wall temperatures and the lowest total thermal resistance among the investigated cases under all examined heating power conditions. Further analysis reveals that the aspect ratio influences vapor transport paths and temperature field uniformity by modifying the annular flow channel geometry; as a/b increases, the flow restriction along the minor axis becomes more pronounced, leading to a slight increase in total thermal resistance. A comprehensive comparison demonstrates that a/b = 1.25 achieves a more favorable balance between flow passage continuity and heat transfer area. These results provide quantitative guidance for the selection of structural parameters and the optimal design of eccentric elliptical cross-sectional radial gravity heat pipes.