NOVELTY - Preparing biofuel by biomass quick thermal cracking process comprises (i) impregnating biomass in diluted acid, filtering, and drying; (ii) subjecting acid impregnated biomass to steam explosion pretreatment to obtain alcohol extract and steam explosion biomass; (iii) adding the biomass with distilled water, glacial acetic acid and sodium chlorite, heating, filtering, and drying to obtain delignified biomass; (iv) obtaining purified cracking vapors; (v) obtaining catalytic esterification thermal cracking vapors; (vi) rapidly condensing; and (vii) distilling, and separating water and biofuel.    USE - The method is useful for preparing biofuel by biomass quick thermal cracking process (claimed).    ADVANTAGE - The method: is simple; produces biofuel with high yield. The biofuel: is oxygen-containing small molecule inflammable organic matter, which can add to diesel oil and gasoline in petrochemical products.    DETAILED DESCRIPTION - Preparing biofuel by biomass quick thermal cracking process comprises (i) impregnating biomass raw material having particle size of 0.5-5 mm in diluted acid at 20-50 degrees C for 0.5-2 hours, where the mass ratio of biomass and diluted acid of 1:(10-50), filtering, drying at 105 degrees C for 3-5 hours to obtain acid impregnated biomass, where the biomass raw material is forest felling residue, forestry processing residues and crop straw, and the diluted acid is sulfuric acid, hydrochloric acid or nitric acid, and the concentration is 0.15-2%; (ii) subjecting the acid impregnated biomass to steam explosion pretreatment by placing in high pressure saturated steam environment at 120-150 degrees C under pressure of 0.6-1.2 Mpa for 30-60 seconds, instantly discharging pressure to atmospheric pressure to obtain steam explosion product, then adding the product with an alcohol solvent, stirring for 30 minutes to obtain alcohol extract and steam explosion biomass, where the alcohol solvent is methanol, ethanol, ethanediol, propanol or butanol, and the mass ratio of the steam explosion product and alcohol solvent is 1:(20-40); (iii) adding the steam explosion biomass with 10-30 pts. wt. distilled water, 0.3-0.5 pts. wt. glacial acetic acid, and 0.3-1 pts. wt. sodium chlorite, heating at 50-70 degrees C for 5-10 minutes, filtering solid substance, washing the solid substance with distilled water until the filtrate is neutral pH 7, drying at 90-105 degrees C for 4-6 hours to obtain delignified biomass; (iv) subjecting the delignified biomass to quick thermal cracking process at 320-380?Oc for 0.5-3 seconds under nitrogen reaction atmosphere to obtain thermal cracking vapors, then subjecting the vapors to solid separation to remove pyrolytic carbon, and obtaining purified cracking vapors; (v) passing the purified thermal vapors through a catalytic reactor from top to bottom, at the same time, continuously spraying and atomizing the alcohol extract in the catalytic reactor, where the catalytic reactor conditions are as follows: catalyst bed layer height is 8-20 cm, reaction temperature 250-280 degrees C, the reaction time is 15-20 seconds, the reaction pressure is 0.8-1.2 Mpa, obtaining catalytic esterification thermal cracking vapors, where the catalyst in the catalyst bed layer is acidic zeolite catalyst; (vi) subjecting the resulting vapors to rapid condensation at 5-10 degrees C to obtain mixture of biofuel and water; and (vii) distilling the mixture of biofuel and water at 40-50 degrees C under relative vacuum degree of -0.08 to -0.095 Mpa, and then separating the water and biofuel.