Abstract: The green and scalable preparation of high-quality graphene and its efficient conversion into thermally conductive polymer composites remain critical challenges for two-dimensional-material-based thermal management. In this work, a urea–ethylene glycol (U-EG) mixed-solvent-assisted high-pressure microfluidization strategy was developed to exfoliate natural flake graphite into high-aspect-ratio few-layer graphene (FLG). The combined shear, turbulent impact, and cavitation effects during microfluidization promoted the progressive exfoliation of graphite, while the U-EG solvent suppressed restacking through hydrogen-bond networks, solvation stabilization, and weak interfacial charge redistribution. Under the optimized conditions, the obtained FLG exhibited few-layer characteristics, a high aspect ratio of approximately 1.0 × 10³, low defect density, and a low oxidation degree, with an actual dry-basis production rate of 4.88 g·h⁻¹. After incorporation into poly(vinylidene fluoride) (PVDF), the thermal conductivity of the composite increased from 0.14 to 1.20 W·m⁻¹·K⁻¹, corresponding to an enhancement of approximately 757%. Model analysis and density functional theory calculations indicate that the enhanced thermal transport originates from the synergistic effect of a continuous FLG-based phonon transport network and stable noncovalent interfacial interactions. This study provides a reference for the green preparation of high-quality graphene and the construction of highly thermally conductive polymer composites.