The development of bio-based liquid fuel is important for the substitution of transport fuels.Fast pyrolysis can efficiently convert solid biomass wastes into bio-oil.However,its poor properties limit its high-quality utilization and therefore upgrading is required.Because of the complicated composition,upgrading bio-oil by a single technique faces many problems,such as low conversion efficiency and severe catalyst deactivation.These problems can be largely overcome by graded upgrading based on separation,in which bio-oil is first separated into several fractions and then these fractions are upgraded by different techniques.This paper is a review of the current states in the upgrading of bio-oil fractions obtained from water extraction and vacuum or molecular distillation.The corresponding upgrading techniques include esterification,steam reforming,catalytic cracking,and hydrodeoxygenation. For each upgrading technique,the corresponding conversion behaviors and mechanisms of typical model compounds in bio-oil fractions are first outlined,and then the applications of this technique in actual bio-oil fraction upgrading are introduced.The graded upgrading of bio-oil based on separation will have more potential and become more economical if various separation techniques,upgrading methods,and the extraction of valuable chemicals can be combined. The development of bio-based liquid fuel is important for the substitution of transport fuels. Fast pyrolysis can fast convert solid biomass wastes into bio-oil. However, its poor properties limit its high-quality utilization and therefore upgrading is required. complicated composition, upgrading bio-oil by a single technique faces many problems, such as low conversion efficiency and severe catalyst deactivation .sese problems can be largely overcome by graded upgrading based on separation, in which bio-oil is first separated into several fractions and then these fractions are upgraded by different techniques.This paper is a review of the current states in the upgrading of bio-oil fractions obtained from water extraction and vacuum or molecular distillation. the corresponding upgrading techniques include esterification, steam reforming, catalytic cracking, and hydrodeoxygenation. For each upgrading technique, the corresponding conversion behaviors and mechanisms of typical mo del compounds in bio-oil fractions are first outlined, and then the applications of this technique in actual bio-oil fraction upgrading are introduced. graded upgrading of bio-oil based on separation will have more potential and become more economical if various separation techniques upgrading methods, and the extraction of valuable chemicals can be combined.