A heat radiation reactor was used to study the mechanism of cellulose rapid pyrolysis in the present paper. Combined with gas chromatography-mass spectrometry analysis on bio-oil, experimental results showed that the production rate of hydroxyacetaldehyde and 1-hydroxy-2-propanone, as well as their proportions in bio-oil, increased with the reaction temperature in the case of short gas residence time. The formation of the above two products was shown to be competitive with levoglucosan formation from active cellulose. In addition, a modified cellulose pyrolysis model based on the Brodio-Shafizadeh model was proposed to describe this competitive phenomenon. Major products' formation was simulated with this modified model, which was in good agreement with the experimental results.