Coke deposits on catalysts during the reforming of biomass pyrolysis volatiles are usually the primary cause of catalyst deactivation and a major challenge for catalyst design. Unlike simple graphite carbon layers, coke deposits consist of complex macromolecular compounds, making it difficult to elucidate their structural features and formation mechanisms at the molecular level. In this work, based on the pyrolysis of wheat straw in a fixed-bed reactor followed by catalytic reforming of volatiles over Fe-Ni catalysts to produce H2-rich syngas, the pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was employed to analyze fragmented molecules derived from the pyrolysis of coke deposits. The chemical structure of coke deposits containing five-membered rings was demonstrated for the first time by backward induction. Fast pyrolysis experiments demonstrated the interactions between primary volatiles and the catalyst, while fixed-bed scaled-up experiments combined with multiphase product analysis demonstrated the role of the catalyst in the formation of the coke deposits. The results showed that the catalyst support guided the adsorption of volatiles and the deposition of carbonaceous species, while metal sites promoted cyclization and polymerization reactions. Coke deposits with different chemical structures contribute differently to catalyst deactivation. Compared to “hard” coke deposits dominated by graphitic carbon, “soft” coke deposits characterized by five-membered ring structures were not the main cause of catalyst deactivation and could continue to convert tar. Model compound studies pinpointed cyclopentenone, a cellulose/hemicellulose pyrolysis derivative, as a key intermediate for the assembly of five-membered ring structures in coke deposits. The reaction pathways and mechanisms of coke formation were further elucidated by combining experimental and theoretical calculations. This study provides some insights for the structural analysis and formation mechanisms of coke deposits on catalysts.