Abstract: The polymers builder of hyperchem and the semiempirical method were used to build and optimize the molecular model of xylan and obtain the characteristic parameter of xylan molecular. Based on Amber force field, in the periodic box of 1.5 nm×1.0 nm×4.5 nm, pyrolytic decomposition of xylan molecular (DP 9) was studied and analyzed under periodic boundary conditions within 300—1 300 K. Results showed that the glycosidic bond broke off at 550 K. At the same time, the “—OH” on pyran ring broke off． When the temperature rose to 650 K, the “C—O” bond of pyranose broke out of the chain, later the “C—C” bond broke down．Through simulation, the main cracking groups were found and the corresponding products were predicted. Meanwhile, the mechanism of productions such as 2 furfuraldehyde, HAA, CO2, CO was also analyzed.