<p>This study employed a self-developed ultrahigh-temperature flash vacuum pyrolysis (UT-FVP) reactor hyphenated with molecular beam sampling (MB) time-of-flight mass spectrometry (TOF-MS) system, achieving the first <i>in situ</i> detection of intermediates during fullerene pyrolysis synthesis. Systematic investigation of methane (CH₄) and its chlorides derivatives (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄) revealed that chlorine atoms play a pivotal role in mediating fullerene formation. Methane and methylene chloride (CH₃Cl) (H/Cl &gt; 1) are hard to generate fullerenes due to insufficient chlorine content. Dichloromethane (H/Cl = 1) exhibited the highest C₆₀ selectivity at about 1600 °C, and its reaction path is dominated by carbonyl alkyne intermediates [(−C≡C−)_<i>n</i>]. While chloroform and carbon tetrachloride systems (H/Cl &lt; 1) generate fullerenes via chlorinated polycyclic aromatic hydrocarbon (PAHs) intermediates (<i>e.g.</i>, C₁₀Cl₈, C₂₀Cl₁₀). To finely regulate the hydrogen-chlorine ratio to verify the chlorine atom-mediated mechanism, CH₂Cl₂/CCl₄ co-pyrolysis experiments were carried out. The results demonstrated that maximum chlorinated fullerene intermediate yields were obtained at an optimized H/Cl ratio of 0.32. The synergistic participation of the dual-carbon source in the construction of the cage-like structure was confirmed by ¹³C labeling. This study reveals the multiple mechanisms of chlorine atoms’ actions by reducing the C–H bond dissociation energy barriers, promoting the exposure of active sites, and regulating the selectivity of reaction pathways. This work establishes novel theoretical bases and process optimization strategies for controlled fullerene synthesis.</p>