NOVELTY - Biological fermentation for producing xylooligosaccharide involves juicing the sorgo straw by an extracting apparatus to obtain the steam explosion straw and rinsed liquid of steam explosion straw; washing the steam explosion straw; carrying out enzymolysis on the steam explosion straw; inoculating the strain to 5-8% fermentation culture to obtain acetone, butanol and ethanol; inoculating the feed strain to the acetone butanol fermentation residue; and decompressing to distil the steam explosion straw liquid at 40-90 degrees C and 0.05-0.2 MPa.    USE - For producing xylooligosaccharide from sorghum; and for producing fermented products of acetone, butanol and ethanol, which are used as biological fuel (claimed).    ADVANTAGE - The process has no waste and pollution discharge; is clean process; and provides improved good social benefit and economic benefit.    DETAILED DESCRIPTION - Biological fermentation for producing xylooligosaccharide involves juicing the sorgo straw by an extracting apparatus, separating the juice from the stem dregs, carrying out steam explosion on the stem dregs of sorgo at 0.5-3 Mpa to obtain the steam explosion straw and rinsed liquid of steam explosion straw; washing the steam explosion straw at 55-60 degrees C for 20-40 minutes; and filtering the rinsed liquid of steam explosion straw; carrying out enzymolysis on the steam explosion straw, where cellulase is added at dose of 5-10 FIU/g straw, pH of 4.8, temperature of 45-55 degrees C, and the enzymolysis time is 20-60 hour; and then mixing the enzymolysis liquid with the sorgo liquid, controlling the sugar content within 6-7%, regulating the pH to 6.5-7 by sodium hydroxide (NaOH), and then adding the nitrogen source to the mixed liquid, where the ratio of the sugar content to the protein content is 15/1-25/1, and then sterilizing for 15-30 minutes at 121 degrees C to obtain the fermentation culture medium; inoculating the strain to 5-8% fermentation culture medium, carrying out anaerobic fermentation for 12-24 hours to obtain the acetone seed liquid, inoculating the acetone butanol seed liquid to the fermentation culture medium based on an inoculation quantity of 5-10%, and then carrying out anaerobic fermentation for 70-75 hours at 37-38 degrees C to obtain the acetone butanol, separating the solid from the liquid after the acetone butanol fermentation to obtain the fermentation liquid and the fermentation residue, measuring the content of the solvent in the fermentation liquid by the gas chromatograph, distilling the fermentation liquid to obtain the acetone, butanol and ethanol; inoculating the feed strain to the acetone butanol fermentation residue; fermenting to produce the feed protein at 28-30 degrees C, the stirring speed is 90-95 revolution/minute, the gas leading quantity is 1/1.5 (vol./vol.), for 48-72 hours, the quantity of the strain after the fermentation is 8-10 g/l, and the crude protein is greater than or equal to 42%; and decompressing to distil the steam explosion straw liquid at 40-90 degrees C and 0.05-0.2 MPa, decoloring the distilled liquid by the resin and active carbon, regulating the pH to 5-6, adding the xylanase based on proportion that each gram of xylan is added with 30-90 UI unit of active xylanase, carrying out enzymolysis for 3-8 hour at 45-60 degrees C, heating to 100 degrees C, keeping for 5-20 minutes to obtain the mother liquid of xylooligosaccharide, and then concentrating in vacuum to obtain the xylooligosaccharide. The nitrogen source is selected from maize pulp, corn gluten, corn gluten powder, bean pulp, bean cake powder, bran, rice bran, yeast powder, peptone, ammonium sulfate, ammonium chloride, ammonium carbonate, ammonium bicarbonate, ammonium dihydrogen carbonate and/or ammonium dihydrogen phosphate.