Abstract: Based on the modified Brodio-Shafizadeh mechanism, a two-stage model was proposed to simulate the formation and decomposition of intermemediate products including active cellulose (AC), levoglucosan (LG), hydroxyl-acetaldehyde (HAA), acetol and furfural in the cellulose pyrolysis. Results show that the water evaporation only decreases the heating rate of the pyrolysis prophase, while it has no effects on the temperature distribution and reaction process in the main pyrolysis stage. Cellulose material stays for a long time in the middle temperature due to the endothermic effect of primary cracking. The concentration of LG and other competitive compounds in the fabric structure is primarily determined by its competitive reaction of formation. The rising of material thickness would prolong the decomposition time of cellulose, and enhance the secondary cracking within the fabric structure. The formation process both LG and HAA shows the characteristic of fast formation and rapid escape, but HAA presents a faster accumulating tendency that will be enhanced at high temperature. For small size materials, the secondary reaction of volatile occurs largely in the gaseous phase rather than in the solid phase. The longer residence time of volatile in the high temperature region will induce deep decomposes of LG. Compared with the rapid decrease of LG yield with reaction time, the yield of tar falls slowly at high temperature. The major change exists in the distribution of components, i.e. the large molecule compounds being decomposed into small ones.