To understanding the biomass pyrolysis process in depth, the pyrolysis mechanism of cellulose was investigated based on the combination of gas and liquid product releasing behavior with the structure evolution of solid product at 200-600 degrees C. In particular, the transformation process of the chemical functional group of solid char was explored with two-dimensional perturbation correlation infrared spectroscopy (2D-PCIS). It was found that at a lower temperature (<350 degrees C), it was mainly the dehydration and keto alcohol isomerization of cellulose and the char was mainly composed of aromatic and alicyclic compounds rich in C=O structures. With temperature increasing (350-450 degrees C), glycosidic bonds were rapidly broken, with volatiles increasing greatly, and formed network structure containing low-order fused rings (2-5 rings). At a higher temperature (450-600 degrees C), the accelerated etherification of pyran rings resulted in a continuous increase of LG and solid char went to higher-order fused rings (2 x 2-4 x 4 rings). These mechanistic insights are helpful for the understanding of the biomass pyrolysis process.