Phase change materials (PCMs) are extensively utilized in solar energy storage, building heating, and waste heat recovery fields, due to their high energy storage density and consistent phase change temperature. However, due to its low thermal conductivity, rapid charge and discharge cannot be achieved. This challenge is typically addressed by incorporating porous media into PCMs. Although the melting characteristics of PCMs in porous media have been widely studied, the flow and heat transfer mechanisms remain inadequately defined, necessitating further research. Therefore, this study simplified the structure of porous media to a rectangular configuration and adjusted the aspect ratio of the pores to analyze the melting characteristics of PCMs at the pore scale. Besides, a factor U was proposed to evaluate temperature uniformity. Combining the maximum temperature analysis, the transient results of the thermal resistance and the evaluation factors were analyzed based on thermal resistance analysis method. The findings indicated that optimizing pore size can significantly enhance heat transfer efficiency, which is crucial for improving the thermal performance of PCMs. Furthermore, the analysis of the structural characteristics of porous media provides a theoretical foundation for understanding the effects of pore structure on fluid dynamics and heat transfer characteristics. The research results not only deepen the understanding of the melting process of PCMs in porous media but also offer a theoretical basis for the structural design of porous media, promoting advancements in related technologies within the fields of thermal energy storage and thermal management.