Abstract: To reveal the microstructure evolution of pyrolysis semi-coke of bituminous coal and its regulatory mechanism on spontaneous combustion response, the synergistic evolution of functional group composition and pore structure of bituminous coal semi-coke at different pyrolysis temperatures was investigated using in-situ Fourier transform infrared spectroscopy combined with macroscopic thermal analysis. The effects of microstructure evolution on the low-temperature oxidation characteristics and spontaneous combustion tendency of bituminous coal semi-coke were also explored. The results show that 600 ℃ is the critical temperature at which the structure of bituminous coal semi-coke transforms from a highly reactive aliphatic-aromatic hybrid to an inert condensed aromatic skeleton. Moderate and low pyrolysis ( < 500 ℃) removes partial volatile matter, while intense pyrolysis breaks chemical bonds and generates numerous unstable aliphatic side chains and free radical sites in the coal matrix. This leads to a remarkably high carbonyl formation rate and aliphatic hydrocarbon consumption kinetics for the 400 ℃ pyrolysis semi-coke at the initial oxidation stage, with significantly enhanced oxidation reactivity. In contrast, high pyrolysis ( > 600 ℃) almost completely removes aliphatic side chains and forms graphite-like crystallites with high thermal stability. The elimination of large numbers of active groups reduces oxygen adsorption and active sites, resulting in decreased oxidation activity. Pyrolysis temperature nonlinearly regulates coal oxidation activity by controlling the dominance between side-chain activation and skeleton polycondensation. Dynamic grey relational analysis based on differential thermogravimetry (DTG) and Pearson matrix analysis quantify the structure-reaction competition mechanism: physical transport of pore structure dominates in the low-temperature induction stage ( < 230 ℃); in the high-temperature active region ( > 280 ℃), the correlation coefficient between aliphatic side chain content and reaction rate rises above 0.80, and specific surface area shows a significant negative correlation with reaction rate. This quantitatively verifies the pore-activity decoupling as a key structure-reaction feature dominated by skeleton polycondensation. Side-chain activation and skeleton polycondensation are two competing and interconverting processes during pyrolysis of bituminous coal semi-coke. Pyrolysis temperature determines the dominant process, which directly controls the microstructure and subsequent oxidation activity of coal.