Abstract: The cotton stalk is a typical biomass waste. Its problems such as the loose structure, low calorific value and low energy density can be effectively solved by hydrothermal treatment and hot pressing forming. In this study, the Xinjiang cotton stalk was taken as the research object, and the molded particles were prepared after completing the 180−280 ℃ hydrothermal treatment experiment using a high-pressure reactor. The diffraction of the main chemical components (cellulose, hemicellulose and lignin) in the cotton stalk was analyzed by means of Fourier infrared analyzer and X-ray diffractometer. The influence of the three-component diffraction behavior of biomass on the physical properties of the molding particles was explored. The influence of the evolution of the main chemical components (cellulose, hemicellulose, lignin) in the cotton stalk on the physical properties of its pellets and the key components were investigated. The results show that with the increase of hydrothermal temperature, the compress strength and apparent density of cotton stalk pellets increased first and then decreased, and reached the highest value at 230 ℃ (The compress strength was 9.3 MPa, the apparent density was 1 478.46 kg/m³). In the hydrothermal treatment process, the hemicellulose was completely decomposed before 200 ℃, the amorphous cellulose and crystalline cellulose were completely decomposed before 230 ℃ and 280 ℃, respectively, while the lignin still existed at 280 ℃. Finally, based on the variation trend of the physical properties of the pellets, it was found that the crystalline cellulose played a skeleton supporting role in the cotton stalk pellets, which was an important component to provide mechanical strength and also an important source of apparent density. Lignin, as a binder in the pellets, could strengthen the degree of bonding between surrounding particles during the densification process.