The solidification behavior of nanoparticle-enhanced composite phase change material (PCM) with internal heat generation is of considerable importance for the design of advanced thermal energy storage and thermal management systems. However, the combined effects of internal heat generation and nanoparticle-induced thermal enhancement on the solidification dynamics in annular configurations have not yet been systematically elucidated. In this study, a theoretical model is established to investigate the Stefan problem associated with the solidification of nanoparticle-enhanced PCM containing uniform internal heat generation in an annular enclosure subjected to convective cooling. Under the small Stefan number assumption, a multi-scale asymptotic analysis is employed to derive analytical solutions for the temperature field, interface evolution, and total solidification time. Four spatial scales and five temporal scales are identified, and a unified mathematical framework is developed by incorporating the effective thermophysical properties of nanoparticle-enhanced PCM based on effective medium theory. The effects of internal heat generation intensity, nanoparticle type, nanoparticle volume fraction, annular geometry, and convective boundary conditions on the solidification process are systematically investigated. The results demonstrate that internal heat generation acts as a volumetric heat source that continuously compensates for heat loss within the solid region, thereby elevating the local temperature, weakening the temperature gradient at the solid–liquid interface, reducing the interfacial heat flux, and delaying the propagation of the solidification front. In contrast, high thermal conductivity nanoparticles significantly enhance the effective thermal conductivity of the composite PCM, accelerate heat transfer, and shorten the total solidification time. Among the three classes of nanoparticles considered, \textCu exhibits the strongest enhancement effect owing to its exceptionally high thermal conductivity, whereas \textAl_2\textO_3 provides the weakest enhancement. A quantitative parametric analysis further reveals a pronounced competitive interaction between internal heat generation and nanoparticle-induced thermal enhancement. As shown in Fig. 11, the total solidification time increases monotonically with increasing internal heat generation intensity but decreases markedly with increasing nanoparticle volume fraction. Even under strong internal heat generation, increasing the volume fraction of copper nanoparticles effectively mitigates the solidification delay, indicating that enhanced thermal conduction can partially offset the adverse influence of internal heat generation. Furthermore, increasing the annular radius ratio and the Bi number significantly accelerates solidification by shortening the heat-conduction path and enhancing convective heat transfer, respectively. These findings clarify the dominant physical mechanism governing the competition between internal heat generation and nanoparticle enhanced thermal conduction during annular solidification. The proposed analytical model provides not only an efficient theoretical tool for predicting solidification behavior but also a quantitative framework for optimizing nanoparticle type, internal heat generation, annular geometry, and boundary heat transfer conditions in composite PCM-based thermal energy storage and thermal management systems.