In order to study the behaviors of waste cotton fiber under hydrothermal conditions, the carbonization process and physicochemical characteristics of waste cotton fiber were explored with microcrystalline cellulose and glucose as models. The structures and properties of different hydrothermally carbonized products were investigated with SEM, XRD, FTIR, XPS and EA while liquid products were characterized by HPLC. The results showed that cotton fiber could form carbon microspheres under hydrothermal conditions. Although the products of cotton fiber had similar crystal structures and oxygen functional groups with the carbonized products of microcrystalline cellulose and glucose, the morphology and properties were worse. Glucose was the key intermediate during the process of carbon microspheres formation from cotton fiber. As cotton fiber had low yield of glucose, only part of that could hydrolysis to simple sugars. These induce two different carbon paths. The main morphology of products is irregular particle. Decreasing the pH of reaction system may promote the cotton fiber carbonized into microspheres.