The precipitation behavior of Cu-rich phases in reactor pressure vessel (RPV) steels under irradiation is a critical factor influencing mechanical property degradation. Understanding the coupled effects of temperature and dislocation defects on phase evolution is essential for predicting long-term material performance. This work employs a three-dimensional aging phase-field model (EPhase) to systematically investigate the nucleation, growth, and spatial distribution of Cu-rich phases in an Fe–15 at.% Cu–3 at.% Mn–1.5 at.% Ni–1.5 at.% Al alloy within the temperature range of 523–923 K, with particular attention to the role of dislocation loops. The results reveal that temperature plays a decisive role in modulating the interaction between dislocations and precipitation, leading to a clear transition in the dominant precipitation mechanism with increasing temperature. At low temperatures (523–623K), precipitation is primarily driven by chemical free energy reduction, with limited sensitivity to the dislocation field. At an intermediate temperature of 723 K, the elastic stress field around dislocations synergizes with thermally activated diffusion, resulting in pronounced spatially heterogeneous precipitation along the dislocation loop. At high temperatures (823–923K), the thermodynamic driving force for precipitation is substantially weakened; Cu-rich phases form only in localized regions where dislocation stress fields provide additional nucleation energy, while precipitation in the dislocation-free matrix is almost entirely suppressed. Energy decomposition analysis further shows that as temperature increases, the contributions of chemical and interfacial energies to the total free energy reduction diminish, whereas elastic energy becomes increasingly dominant, ultimately inhibiting both nucleation and growth of Cu-rich phases. Correspondingly, the precipitation strengthening effect degrades significantly with increasing temperature. At low temperatures, a high number density of finely dispersed Cu-rich precipitates leads to a substantial increase in yield strength. At elevated temperatures, however, precipitates are coarser, fewer in number, and spatially localized, offering little resistance to dislocation motion and resulting in negligible strengthening. These findings elucidate the coupled role of temperature and dislocation loops in governing the precipitation behavior of Cu-rich phases and provide a theoretical basis for predicting microstructural evolution and mechanical performance in RPV steels under long-term service conditions.