Bio-tar, a byproduct of biomass pyrolysis, poses environmental and processing challenges owing to its tendency to clog pipelines and its ecotoxicity. Converting bio-tar into functional carbon materials offers a sustainable route for waste valorization; however, the underlying thermal polymerization mechanisms remain poorly understood. Herein, we present a single-functional model-compound-assisted analytical strategy to elucidate reaction pathways and polymerization mechanisms in multifunctional group coupling systems. By constructing a model bio-tar (M-bio-tar) that reflects the chemical heterogeneity of real samples, we uncover a temperature-dependent, stage-specific polymerization mechanism comprising volatile release (≤ 200 °C), radical-driven crosslinking polymerization (200-400 °C), and carbon skeleton consolidation (≥ 300 °C). Radical dynamics involving alkyl and hydroxyl radicals (R^• and HO^•) are key contributors to crosslinking processes, while oxygenated intermediates, such as aldehydes and furans, enhance polymerization efficiency via synergistic Diels-Alder and cyclization reactions. Structural evolution analyses reveal temperature-dependent trade-offs among graphitization, dehydrogenation, and porosity development. Temperature-mediated graphitization and heteroatom elimination result in bio-carbons with tunable physicochemical properties. Thermodynamic calculations support the proposed oxygen-regulated reaction pathways and reveal the catalytic roles of unsaturated functionalities. These findings establish a mechanistic framework for engineering bio-tar-derived carbon materials that integrates biomass utilization and advanced material design, thereby advancing the rational development of sustainable carbon materials for energy and environmental applications within a circular bioeconomy.