Abstract: Abstract: It is important to fully understand the formation mechanism and the physicochemical characteristics evolution of the microwave-assisted hydrothermal products from crop residues, which is also of great significance for high added value utilization of crop residues and quality control of hydrothermal products. The control variable method was used to design the single factor experiment. The effects of hydrothermal temperature, retention time, catalyst and crop residue types on the compositions and structural characteristics of the microwave-assisted hydrothermal products from rice straw, corn stover, rape stalk and cotton stalk were studied. The results showed that with the increase of hydrothermal temperature and retention time, the pH value of the liquid products decreased first and then increased, reaching the lowest value of 3.13, while the electrical conductivity and PO43(-P concentration of the liquid products increased first and then decreased, and the NH4+-N concentration of the liquid products increased and reached the highest concentration of 155 mg/L at 260 ℃. With the increase of hydrothermal temperature and retention time, the yields, H/C and O/C of hydrothermal cokes decreased, while the fixed carbon, C and higher heating value (HHV) of hydrothermal cokes increased. The addition of alkaline catalyst of K2CO3 decreased the C content, HHV value, carbon conversion rate and energy conversion rate of hydrothermal cokes from crop residues, while increased the O/C content of hydrothermal cokes. The carbon conversion rates and energy conversion rates of hydrothermal cokes from crop residue can reach 56.65%-98.13% and 58.22%-92.19%, respectively. With the increase of hydrothermal temperature and retention time, the fragmentation degree in the surface and interior of hydrothermal cokes of the four crop residues were getting more seriously, and the surface and interior of hydrothermal cokes exhibited more nano carbon microsphere structures, while the O-H bond of the hydrothermal cokes of the four crop residues increased first and then decreased. Higher hydrothermal temperature increased the aromatic hydrocarbon structures of C=C, C-H, C-C and active oxygen functional groups of C=O and C-O of hydrothermal cokes, while the aromatic hydrocarbon structures and active oxygen functional groups of hydrothermal cokes increased first and then decreased with the increase of retention time. With the increase of hydrothermal temperature and retention time, the specific surface area, pore volume and pore diameter of hydrothermal cokes from crop residue increased first and then decreased. The addition of alkaline catalyst of K2CO3 increased the aromatic hydrocarbon structures, active oxygen functional groups, specific surface area, pore volume and pore diameter of hydrothermal cokes from crop residue. Comparing the four types of hydrothermal cokes from crop residues, the yields of hydrothermal cokes from cotton stalk and rice straw were higher, and the contents of C, H, volatile matter and the higher heating value of hydrothermal coke from rape stalk were the highest. The nano carbon microsphere structures of hydrothermal cokes from corn stover, rice straw and rape stalk were relatively obvious. Comparing the hydrothermal cokes from the other three types of crop residues, the specific surface area and pore volume of hydrothermal coke from cotton stalk were the largest, while the pore diameter was the smallest.