Abstract: Hydrothermal depolymerization of lignin were conducted in catalytic system of formic acid/formate and Ru/C at 275-350℃. The effects of hydrogen donator type and dosage, temperature, different fractions of alkali lignin on hydrothermal depolymerization were investigated by GC-MS and GC-FID. The results showed that the best catalytic effect with highest liquid product yield was realized when the degradation was conducted in the co-catalyst system of formic acid and Ru/C, and the component of liquid product (LP) was relatively simple. The yield of LP achieved the maximum when the dosage of formic acid was 0.8 mol/L. Furthermore, as the most monomer phenols in LP, 4-methyl guaiacol gained a 32.77% share. With the rising reaction temperature, the yield of LP increased at first and then decreased, and peaked at 325℃. L1, L2 and L3 were three different fractions of alkali lignin. Among them, L1 made the 61.80% contribution to the yields of LP , and the maximum total yield of main monomer phenols, i.e., 112.71 mg/g, was achieved. Among the monomers, guaiacol and 4-methy-guaiacol were dominant components, and represented 35.38 and 35.52 mg/g, respectively. Proper isolation of lignin before hydrothermal conversion was benefit for the liquefaction reaction.