The direct deoxygenation effect of CaO on bio-oil from cellulose pyrolysis was studied in a series of drop tube experiments together with gas chromatography-mass spectrometry analysis. The oxygen contents of the bio-oils were found to decrease from 44.4% for CaO-free cellulose to 40.7% when the CaO/cellulose mass ratio (R) was 2. The relative variation of the oxygen content of the bio-oils reached -8.4% for this mass ratio. The relative abundance of levoglucosan decreased sharply from 85.72 to 42.75% with an increase of R from 0 to 2. The decrease of the relative abundance of levoglucosan and an increase of the relative abundance of acetol and other small molecule products indicated a dominant shift of the parallel competitive reactions from the glycosidic bond rupture pathway to the decomposition and reforming route. The direct fixation of active quasi-CO2 intermediates, functioning as a 'chemical sink' in the decomposition and reforming route, caused this shift.