To figure out the role of CaO (calcium oxide) on WGS (water gas shift) reaction and the related enhancing mechanism, the WGS reaction of absorption enhanced biomass steam gasification with CaO addition was performed with thermodynamic simulation and bench-scale experiment. The results showed that, the experimental WGS reaction was kinetically limited at lower temperature and far from equilibrium without CaO addition. However, much higher H-2 concentration could be obtained with CaO addition, as CaO acted not only as a CO2 absorbent, but also a catalyst for WGS reaction. And the enhancing effect showed obvious segmentation feature with the reaction time streamline due to the variation of CaO carbonation performance. Typical product gas composition in stable region (10-70 min) was approximately 71 vol% H-2, 22 vol% CO and 7 vol% CO2, with CO2 absorption rate of 87.41%-91.44%, CO conversion rate of above 75.5%, and enhancement ratio of CaO of about 81%. The possible catalytic mechanism for absorption enhanced WGS reaction could be concluded as carboxyl mechanism, while CaO activated H2O dissociation forming H* and OH*, thus further reacted with CO* to generate COOH* and then decomposed to CO2 and H*, and two H* met to form H-2. The H2O/CO increasing was favorable for WGS reaction. (c) 2014 Elsevier Ltd. All rights reserved.