Abstract: The advent of the 5G era has brought severe heat-dissipation challenges to electronic devices owing to their miniaturization and integration. Polymer matrix composites, featuring lightweight, easy processability and intrinsic electrical insulation, have become a key research subject in thermal-management materials. Among various strategies, the pre-construction of three-dimensional (3D) thermally conductive networks stands out by building continuous phonon transport pathways. It effectively circumvents the inherent limitations of conventional systems with randomly dispersed fillers, such as high interfacial thermal resistance and large percolation thresholds. This strategy greatly improves thermal conductivity while retaining outstanding electrical insulation, and delivers multiple synergistic functions. This review first systematically reviews two typical strategies for enhancing the thermal conductivity of polymers. It further analyzes polymer thermal-conduction mechanisms and key influencing factors including filler dispersion and orientation, and clarifies the superiority of the 3D-network strategy over conventional filling approaches. Emphasis is placed on comparing the synergistic mechanisms of fillers of different dimensions, as well as critically assessing the merits, drawbacks and technical applicability of mainstream network fabrication techniques: freeze-drying, template method, chemical vapor deposition and self-assembly. Finally, we summarize research advances in the multifunctional integration of insulating highly thermally conductive polymers fabricated via 3D network construction. Remaining challenges and future research prospects in this field are also discussed. This review intends to offer a systematic reference for structural design and performance optimization of high-thermal-conductivity insulating composites.