We herein investigate the various roles of calcium oxide in the pyrolysis of biomass at a variant temperatures. The evolution of pyrolysis products was examined to propose the various roles of Ca at a range of temperatures and CaO addition ratios with cotton stalk on a fixed-bed reactor. We found that upon the addition of CaO, the content of ketones produced increased, while that of acidic compounds decreased. Under similar conditions, the concentration of evolved H₂ and CH₄ increased, while that of CO₂ decreased. Thus, variation in the CaO/biomass (Ca/B) mass ratios and pyrolysis temperatures indicated that CaO could act as a reactant, an absorbent, and a catalyst at Ca/B ratios of <0.2, >0.2, and >0.4, respectively. Moreover, at temperatures >600 °C, the roles of CaO as an absorbent and a reactant were less apparent, while its role as a catalyst was enhanced.