The invention relates to the biomass thermal cracking liquefying fuel oil technology, which comprises the following process steps: (1) preprocessing biomass, namely preparing soak solution from water, 5 mol/L of hydrochloric acid and hydrofluoric acid in a volume ratio of 20:1:0.5, soaking for 30 minutes, and removing ash content; (2) performing thermal cracking of the biomass; and (3) refining biological oil, namely performing catalytic cracking on the prepared heavy biological oil, preheating the heavy biological oil to the temperature of 400 DEG C, and keeping the temperature of between 410 and 490 DEG C in a fixed bed reactor, wherein a catalyst is shape-selective molecular sieve based catalysts zeolite socony mobile-5 (ZSM-5), Na-ZSM-5, hydrogen zeolite socony mobile-5 (HZSM-5) or an aluminum phosphate molecular sieve, the feed rate is controlled at 3 to 6 m<3>/h according to the size of equipment, and the biological oil is catalyzed and subjected to the thermal cracking under the condition that the using amount of the catalyst is 2 percent of the mass of the biological oil. In the biomass thermal cracking liquefying fuel oil technology, the ash content is removed, and the influence of the ash content is reduced in the thermal cracking process, so that the yield of the biological oil is improved by 50 to 60 percent. In addition, the biological oil is refined by a catalytic cracking method after cracking reaction, and oxygen in the biological oil is removed in form of CO, CO and HO by the catalyst at intermediate temperature under normal pressure in a thermochemical method; and thus, the biological oil is converted into liquid fuel which is stable, high in oil quality and high in energy density and can be applied widely and directly at normal temperature.