Molecular dynamics simulation method, based on the force field, is an important and effective method to study the structure and performance of molecule. For the purpose of understanding of formation mechanism of major products from the thermal decomposition of cellulose，this paper investigates the pyrolysis mechanism of cellulose with 10 monomers by molecular dynamics simulation. The simulation results show that hydroxide radical is produced through COH bond scission at about 450 K; with the increase of temperature, at about 600 K cellulose monomer gets to be formed by glucoside bond rupture, at the same time, pyranoid ring is opened and all kinds of molecular fragment are formed. Based on the other experimental results of references, the possible formation paths of major products through reaction of all kinds of molecular fragment are analyzed.