•The effect of CaO on volatiles gasification was studied under different conditions.•The experiment was performed in a 2-stage fixed-bed pyrolysis–gasification system.•CaO acts both as a CO2 absorbent and a catalyst in the gasification process.•Temperature of 600–700 °C and water injection rate of 0.1 g min−1 is appropriate.•The H2 yield is closely related to the volatile content of biomass materials.The effect mechanism of calcium oxide (CaO) addition on gasification of pyrolytic volatiles as a key sub-process in the absorption-enhanced steam gasification of biomass (AESGB) for H2 production at different conditions was investigated using a two-stage fixed-bed pyrolysis–gasification system. The results indicate that CaO functions as a CO2 absorbent and a catalyst in the volatiles gasification process. CaO triggers the chemical equilibrium shift to produce more H2 and accelerates volatile cracking and gasification reactions to obtain high volatile conversion rates. Increasing the gasification temperature could improve the reaction rate of cracking and gasification of volatiles as well as the catalytic effect of CaO, which continuously increase H2 yield. When the gasification temperature exceeds 700 °C, the sharp decrease in CO2 absorption capability of CaO drastically increases the CO2 concentration and yield, which significantly decrease H2 concentration. The appropriate temperature for the absorption-enhanced gasification process should be selected between 600 °C and 700 °C in atmospheric pressure. Increasing the water injection rate (represented as the mass ratio of steam to biomass) could also improve H2 yield. The type of biomasses is closely associated with H2 yield, which is closely related to the volatile content of biomass materials.