Abstract: Hydrothermal carbonization is considered as one of the most promising pretreatment technologies for biomass. Temperature is the most important parameter during hydrothermal pretreatment. To explore the feasibility of improving the quality of biomass pyrolysis products,especially bio-oil quality by adopting hydrothermal pretreatment,the properties of hydrochar and its pyrolysis characteristics prepared from corn stalk at different hydrothermal temperatures from 160 ℃ to 220 ℃ were researched using a thermogravimetric analyzer and a fixed-bed experiment device respectively. The bio-oil composition was analyzed by a gas chromatograph-mass spectrometer and the peak area normalization method was used to calculate the relative content of main organic matter. The results show that hydrothermal pretreatment can significantly remove hemicellulose and ash content from corn stalks,especially some metal elements,such as K,Na and Mg,while changing the pyrolysis characteristics of cellulose and lignin,including promoting the conversion of cellulose to sugar and lignin to solid product. With hydrothermal temperature increasing,the yields of bio-oil from hydrochars pyrolysis at 550 ℃ first increase and then decrease. The hydrochar obtained at 180 ℃ has the maximum bio-oil yield of 64%,meanwhile the relative content of sugars in the bio-oil is 46.4%,which is beneficial to high-value utilization of bio-oil.