Methane plasma pyrolysis represents a promising clean technology for solid carbon production, enabling simultaneous hydrogen generation without direct CO₂ emissions. Current research primarily focuses on single-feed methane processes, yet the strategic utilization of byproduct acetylene to enhance reaction efficiency and product value remains underexplored. This study systematically optimized key parameters—including plasma power, methane flow rate, and nozzle design—achieving a methane conversion rate of 91.3% and a solid carbon selectivity of 21.9%. Furthermore, an innovative cyclic feeding strategy combining methane and in-situ-generated acetylene was proposed. This approach significantly boosted solid carbon selectivity to 36.9%, outperforming conventional pure methane feeding. Mechanistic analysis revealed that acetylene not only redirects reaction pathways toward solid carbon formation but also acts as an efficient carbon source and template, promoting the synthesis of highly graphitized solid carbon with a graphitization degree of 89.2%. These findings provide critical insights into plasma-driven carbon material synthesis and establish a foundation for sustainable manufacturing through byproduct valorization.