NOVELTY - Process of preparing biodiesel comprises: adding oil, low-carbon alcohol carbon solid acid catalyst into the three-necked container with electromagnet stirring, thermometer and reflex condensing pipe, heating, refluxing and stirring for 1-10 hours; filtering, separating the filtrate; adding sodium bicarbonate (NaHCO3) solution; cleaning the mixed solution; separating organic phase; drying; after decompressing and distilling, obtaining light yellow transparent biodiesel as product.    USE - Process for the preparation of biodiesel using carbon solid acid as catalyst (claimed).    ADVANTAGE - Preparation of biodiesel using carbon solid acid as catalyst is high yielding, has warm reaction condition, short reaction time, has no environmental pollution and saponification side reaction, and has lower requirement of raw material; and catalyst has low cost, has little consumption and can be repeatedly used.    DETAILED DESCRIPTION - Process of preparing biodiesel comprises: adding oil, low-carbon alcohol carbon solid acid catalyst into the three-necked container with electromagnet stirring, thermometer and reflex condensing pipe, heating, refluxing and stirring for 1-10 hours; heating is controlled at 50-110 degrees C; filtering the reacted material, filtering out the catalyst and filtrate, separating the filtrate into glycerine at the upper layer; NaHCO3 solution with 5-10% of the quality content is added into the upper mixed solution; the adding amount of NaHCO3 solution is 2-6 times of the upper mixed solution; the mixed solution is cleaned until the pH value is neutral; the organic phase is separated by the tap funnel; drying the organic phase by anhydrous magnesium sulfate (MgSO4) solution and filtering the solution to remove the desiccating agent; after decompressing and distilling, obtaining light yellow transparent biodiesel as product. The quality ratio of the added oil, low-carbon alcohol carbon solid acid catalyst is 1:0.1 to 0.5:0.04 to 0.1. Oil is plant oil, animal oil, waste oil or hogwash oil in manufacturing course. Lower carbon alcohol is methanol, ethanol, propanol, or butyl alcohol. Carbon-group solid acid catalyst is polycyclic aromatic hydrocarbon or mixtures, and concentrated sulfuric acid or fuming sulfuric acid are heated and reacted; the sulfuric acid is vaporized out through decompression; after washing and drying, the carbon-group solid acid is obtained. The specific steps involves: mixing polycyclic aromatic hydrocarbon or mixture and concentrated sulfuric acid or fuming sulfuric acid, stirring for 4-24 hours at 180-280 degrees C, decompressing and vaporizing to remove sulfuric acid, washing in hot water at 80 degrees C, placing into oven at 100 degrees C to dry for 6-12 hours; and the carbon-group solid acid is obtained. The polycyclic aromatic hydrocarbon or mixture is naphthalene, methylnaphthalene, anthracene, phenanthrene, tar, 9C fraction, or asphalt; the concentration of concentrated sulfuric acid is greater than or equal to 96%; the quality ratio of polycyclic aromatic hydrocarbon or the mixture and concentrated sulfuric acid or fuming sulfuric acid is 5-20:100.