Abstract: Waste cotton fibers were used as the raw material to prepare carbon microspheres by treatment with subcritical water. Results show that the cotton fibers are hydrolyzed, dehydrated and carbonized to form carbon spheres with a graphitic structure in subcritical water with different pH values. The carbon spheres prepared at a pH value of 3.5 have particle sizes of 0.8-3 μm and a carbon content of 74.99%. High temperature and pressure lead to the destruction of the crystal structure of the cotton fibers. The β-1,4 glycosidic linkage in a cellulose molecule is attacked by H⁺ during hydrolysis. The smaller the pH value, the higher the glucose yield in hydrolysis, and the more regular the morphology of the carbon spheres, the greater the carbon content and calorific value of the carbonized products. It is difficult to prepare spherical carbons under high pH values due to the low hydrolysis yields of glucose.