Despite extensive research, our understanding of the dolomite calcination mechanism remains unclear, especially concerning how dolomite calcination is influenced by a change in H2O and CO2 partial pressure under high-pressure conditions. In this study, dolomite calcination behaviors and mechanisms at different H2O and CO2 partial pressures were investigated using thermogravimetric analysis, differential scanning calorimetry, and scanning electron microscopy. Under dry thermal conditions, CO2 has a delaying effect on dolomite calcination; however, this effect is independent of CO2 partial pressure, indicating that the calcination could be controlled by the CO2 adsorption capacity on dolomite active sites. In an H2O atmosphere, calcination might begin at a low temperature due to the adsorption of H2O on active sites and then be controlled by the dissociation of HCO3−. A delaying effect of H2O was also observed, with the H2O partial pressure being lower than the CO2 partial pressure in an H2O and CO2 mixture. This could be attributed to the formation of CO32− via 2OH− + CO2 = CO32− + H2O. A model was developed to predict the dolomite conversion time under isothermal conditions. A process window considering the effect of operating variables on the conversion time at 550–1000 °C and H2O partial pressures of 1–20 bar is presented.