NOVELTY - Preparing biodiesel by fermenting cellulose, comprises taking vegetable fiber degrading bacteria and oil-producing bacteria into potato dextrose agar culture medium, adding oil-producing bacteria and vegetable fiber degrading bacteria into fermentation culture medium, filtering and containing the culture solution, filtering the oil producing microorganism, grinding the oil-producing microorganism cell, adding extractant with grinded oil-producing microorganism cell, extracting and evaporating the solvent with the mixture and performing microorganism oil using methyl esterification reaction.    USE - The method is useful for preparing biodiesel oil.    ADVANTAGE - The method is capable of economically preparing the biodiesel oil with reduced fuel consumption in an environment-friendly manner.    DETAILED DESCRIPTION - Preparing biodiesel by fermenting cellulose: (a) taking vegetable fiber degrading bacteria and oil-producing bacteria into potato dextrose agar culture medium at 25-30 degrees C for 3-5 days to prepare a microorganism seed liquid; (b) adding the oil-producing bacteria and the vegetable fiber degrading bacteria into a fermentation culture medium in a cell weight ratio of 2-3:1, and shaking and cultivating the mixture under a speed of 180 rpm at 28 degrees C for 5-14 days to prepare a mixed fermentation culture medium, where the mixed fermentation culture medium comprises culture medium I and/or culture medium II, the culture medium I comprises 10-50 g of cellulosic material, 9 g of disodium hydrogen phosphate dodecahydrate, 1.5 g of potassium hydrogen phosphate, 1.2 mg of ferric ammonium citrate, 0.34 g of ammonium sulfate, 0.15 g of peptone, 0.15 g of yeast extract, 0.3 g of calcium chloride, 0.3 g of magnesium sulfate, 0.005 g of ferrous sulfate heptahydrate, 0.0016 g of manganese sulfate monohydrate, 0.0014 g of zinc sulfate monohydrate, 0.002 g of phosgene and 1000 ml of water, the culture medium I is a faintly acid to adjust in a pH value of 6-6.8, and the culture medium II comprises 10-50 g of cellulosic material, 2 g of potassium hydrogen phosphate, 1.4 g of ammonium sulfate, 0.3 g of magnesium sulfate, 0.3 g of calcium chloride, 0.5 g of peptone, 0.005 g of ferrous sulfate heptahydrate, 0.0016 g of manganese sulfate monohydrate, 0.0014 g of zinc sulfate monohydrate, 0.002 g of phosgene and 1000 ml of water, the culture medium II is pH is the faintly acid to adjust in a pH value of 6-6.8; (c) filtering the culture to remove the cellulose degrading bacteria and residue of fermentation liquor to get culture solution, containing the culture solution using culture medium to obtain oil producing microorganism, and filtering the oil producing microorganism to get oil-producing microorganism cell; (d) grinding the oil-producing microorganism cell using a high pressure homogeniser, adding extractant with the grinded oil-producing microorganism cell, collecting and extracting an upper organic solvent layer with the mixture, and evaporating and recovering solvent with the extracted mixture to get remaining part of microorganism oil; and (e) performing the microorganism oil using a methyl-esterification reaction to obtain the end product.