Enhanced antioxidant activity of aqueous phase bio-oil by hydrothermal pretreatment and its structure-activity relationship

•Aqueous phase bio-oil (APBO) was obtained from hydrothermal pretreatment combined with pyrolysis of biomass.•Hydrothermal pretreatment enhanced the relative content of phenolic compounds and the antioxidant activity of APBO.•APBO exhibited excellent antioxidant activity, which affected by its chemical components.•Phenols exhibited the promotion effect on antioxidant activity of APBO, while acids showed the inhibition effect.•APBO has the potential to be used as the alternative feed additives and alternative antioxidant.