The invention relates to a method for preparing bio-oil through online deoxygenation and hydrogenation of biomass thermal cracking liquification, belonging to the technical field of biomass energy. The method comprises the steps of firstly enabling a biomass raw material to react with calcium formate to obtain a mixed sample; performing a thermal cracking liquification reaction on the mixed sample to obtain a reaction product comprising bio-oil, un-condensable gas and a mixture of pyrolytic carbon and calcium carbonate; performing high-temperature gasification on the mixture of pyrolytic carbon and calcium carbonate to obtain carbon monoxide and calcium oxide; adding water into calcium oxide, and obtaining calcium hydroxide through a nitration reaction; performing a carbonylation reaction on calcium hydroxide and carbon monoxide to generate calcium formate, wherein the generated calcium formate is used as an auxiliary raw material for a thermal cracking liquification reaction. The method provided by the invention is simple in preparation process and improves the product performance without increasing the production cost; by adopting the auxiliaries such as formate and the like as oxygen supply carriers, the production cost is reduced. By adopting the method, the oxygen content in the prepared bio-oil is remarkably reduced, and the stability and heat value of the bio-oil are improved.