Abstract: Hydrothermal liquefaction of cellulose under different atmospheres with and without catalyst was performed in a 500 mL high pressure stirred reactor. The results indicated that without catalyst the effect of different atmospheres was weak. The conversion of cellulose was from 77.67% to 81.16% and the yield of liquid products was from 16.22% to 18.61%. No significant activity of reductive atmosphere (H₂ and CO) can be determined under this condition. By adding KOH as catalyst, the reducing atmosphere was favorable to improve the conversion of collusion and the yield of liquid products. Especially under CO atmosphere, the conversion of cellulose, the yield of liquid products and the heating value reached 98.49%, 33.33% and 38.98 MJ/kg, respectively. This should relate to the strengthening of the direct CO-deoxygenating and depolymerization of cellulose by the active hydrogen produced by CO transformation catalyzed by KOH.