Establishing a direct relationship between the mass change of iron-based oxygen carrier and gas production is crucial for the efficient operation and the control of the lattice oxygen release in biomass chemical looping gasification. In this study, a thermogravimetric fixed-bed reactor was designed to achieve the simultaneous measurement of the mass change and gas composition in the reduction of the oxygen carrier. Three oxygen carriers of pure iron (III) oxide, natural hematite, and red mud were used as candidates. The phase transfer of the oxygen carrier along with the biomass pyrolysis/gasification was studied. The mass loss rates and the gas composition were matched by three significant phase transition stages. The phase transitions from Fe2O3 to Fe3O4, Fe3O4 to FeO, and FeO to Fe occurred when CO appeared, CO/CO2 = 0.97, and CO/CO2 = 2.61, respectively. The extent of oxygen carrier reduction was 28.61% of pure iron (HI) oxide, whereas the oxygen carrier reduction molar ratio was 74.50% of red mud. X-ray diffraction (XRD) spectrums showed the oxygen carrier was reduced to the metallic iron phase. Sintering and agglomeration were found in the reacted oxygen carriers of the scanning electron microscope (SEM) images.