Abstract: According to the experimental result on cellulose pyrolysis in heat radiation reactor, a mechanism of cellulose pyrolysis was studied on primary products formation, such as levoglucosan, glycolaldehyde and acetol etc. In the case of rapid devolatilization and following chill, the formation of glycolaldehyde and acetol appeared to compete with that of levoglucosan. Then, a kinetic model based on competitive reaction was proposed to simulate the formation of glycolaldehyde and acetol. It was concluded that formation of glycolaldehyde was similar with that of acetol in kinetic behavior, showing that there existed two similar initial formation reactions. During pyrolysis process, cellulose underwent decomposition through heterolytic or homolytic fission of some reaction precursors, which could be called as active cellulose. Anhydrosugars, including levoglucosan and its isomeric compounds, were formed by heterolytic fission of glucosidic bond under transglycosylation reaction. While small molecular compounds, such as glycolaldehyde, acetol and CO, were formed by acetal open-loop and following C-C bond rupture. On the basis of Brodio-shafizadeh model, a mechanism model was proposed to simulate levoglucosan formation at high temperature of thermal radiation source. The calculation result was consonant well with experimental value.