Pyrolysis is a very important platform for the use of cellulose; however, the mechanism for the pyrolysis of cellulose is still not clear. A density functional theory (DFT) study was thus conducted to investigate the dehydration of cellulose using cellotriose as a model compound. The DFT study confirms that the location of hydroxyl groups has a significant influence on the dehydration of cellulose. The most active hydroxyl group is -O2H, followed by -O3H and -O6H. The pinacol rearrangement is a more likely mechanism for hydroxyl groups to be dehydrated. However, the barriers for dehydration are so high that dehydration reactions are difficult to occur. To obtain chemical products directly from the pyrolysis of cellulose, species with hydroxyl groups remaining might be targeted. Future studies are needed to explore the mechanism of cellulose pyrolysis, especially on the breaking of the glycosidic linkage and the opening of pyran rings.