The release mechanism of Ca during coal pyrolysis and char gasification in H2O, CO2 and their mixtures was studied. Sequential chemical extraction was used to determine the modes of occurrence of Ca in coal and char. The released Ca from coal pyrolysis and char gasification were captured and analyzed by activated carbon adsorption and X-ray photoelectron spectroscopy (XPS). Model compounds CaS and CaSO4 were adopted to further reveal the released form of Ca under different atmospheres. The results indicate that Ca in coal is mainly released as CaCl2 during the pyrolysis process, and the possible migration mechanism of Ca during pyrolysis was proposed. Ca in coal is mainly released in the form of CaCl2, CaCO3, and CaSO4 during the gasification, and Ca is released as CaCl2 under all conditions. In addition, Ca will be released as CaCO3 under CO2 atmosphere, as CaSO4 under H2O and H2O/CO2 atmospheres at 800 °C and 900 °C, released as CaSO4 under all conditions at 1000 °C. This is closely related to the formation of CaO2 intermediates during the gasification process.