Abstract: The mechanism involved in the phase transformation process of pyrolusite (MnO₂) during roasting in a reducing atmosphere was systematically elucidated in this study, with the aim of effectively using low-grade complex manganese ore resources. According to single-factor experiment results, the roasted product with a divalent manganese (Mn²⁺) distribution rate of 95.30% was obtained at a roasting time of 25 min, a roasting temperature of 700°C, a CO concentration of 20at%, and a total gas volume of 500 mL·min⁻¹, in which the manganese was mainly in the form of manganosite (MnO). Scanning electron microscopy and Brunauer–Emmett–Teller theory demonstrated the microstructural evolution of the roasted product and the gradual reduction in the pyrolusite ore from the surface to the core. Thermodynamic calculations, X-ray photoelectron spectroscopy, and X-ray diffractometry analyses determined that the phase transformation of pyrolusite followed the order of MnO₂→Mn₂O₃→Mn₃O₄→MnO phase by phase, and the reduction of manganese oxides in each valence state proceeded simultaneously.