The thermochemical utilization of biomass char is vital for addressing environmental pollution and the energy crisis. The carbonaceous structure plays a crucial role in this process, and understanding the conversion mechanism aids in optimizing biomass reactors. This study performs a comprehensive analysis of hightemperature rice husk char using 13C NMR, XPS, and FTIR techniques. The carbonaceous structure is classified based on carbon bonding modes and the forms of O and N. The reaction mechanisms of thermochemical transformations under various atmospheres (N2, O2, CO2, H2O) are investigated using ReaxFF-MD. The reaction characteristics are determined, and the reaction activities are compared. The results indicate that the nitrogencontaining heterocycles remain in pyridine-type pyrolysis products under the N2 atmosphere (pyrolysis). Repolymerization occurs in the initial stages of pyrolysis, with the size of aromatic rings affecting the formation of heavy components. Under the O2 atmosphere (combustion), the degree of complete carbon oxidation is linked to the number of aromatic rings. The reactivity of aliphatic carbonaceous structures depends on the hydrogen content. Under the CO2 atmosphere (gasification), the pyrrole-type structure exhibits the highest gasification activity. Aromatic structures with oxygen functional groups are more susceptible to reacting with CO2, indicating higher sensitivity. Aliphatic H atoms tend to be oxidized into H2O. Under the H2O atmosphere (gasification), hydrogen in the gasification of aromatic carbon primarily originates from H at the aromatic boundary. In N2 and CO2 atmospheres, the C-O bridge bond breaks first, followed by reactions. In H2O gasification, the breaking of bridge bonds connected to aliphatic carbon occurs simultaneously with gasification. Each reaction system’s product distribution and elemental migration processes are also determined. The findings provide valuable theoretical insights into the application of biomass char.