NOVELTY - One-step method for preparing biodiesel fuel using biomass based sulfonated carbon as catalyst for fatty acid esterification and triglyceride transesterification comprises (a) steeping biomass with rich lignin, draining, carbonizing, collecting, washing, drying, and sealing; (b) putting degummed raw oil, biomass based sulfonated carbon, and low carbon alcohol in methanol rectification water separation device, heating, reacting during stirring; and (c) separating product and cooling, filtering, recycling, regenerating, and distilling the filtrate.    USE - The process is useful for preparing biodiesel fuel using biomass based sulfonated carbon as catalyst for fatty acid esterification and triglyceride transesterification (claimed).    ADVANTAGE - The process solves problem of traditional acid and base catalysis, including limitation caused by acid value of raw oil and large discharge amount of waste water. The catalyst is insensitive to water in side product during production of biodiesel fuel, and is easy to separate, which is favorable for purifying rough glycerin. The catalyst has low cost, can be regenerated and used as material for improving soil.    DETAILED DESCRIPTION - One-step method for preparing biodiesel fuel using biomass based sulfonated carbon as catalyst for fatty acid esterification and triglyceride transesterification comprises (a) steeping 100 parts of biomass which rich lignin by 300 parts of diluted sulfuric acid overnight, taking the biomass out and draining, carbonizing in low temperature gas retort for 1-6 hours, collecting acetic acid, furfural and methanol by condensing, washing the carbide by de-ionized water until no dissociative sulfuric radical exists, drying at 105 plus minus 5 degrees C for 1-2 hours by air dry oven, which forms 70-85 parts of porous biomass based sulfonated carbon, sealing; (b) putting 100 parts of degummed raw oil, 5-10 parts of biomass based sulfonated carbon, 100-300 parts of low carbon alcohol in methanol rectification water separation device, heating to 65-120 degrees C, reacting during stirring for 1-8 hours; and (c) separating product by placing it still and cooling, where the upper layer is biodiesel fuel and lower layer is mixture of excess low carbon alcohol, glycerin and biomass based sulfonated carbon, treating the upper layer by vacuum rectification, filtering the lower layer, recycling the biomass based sulfonated carbon, regenerating it, or using it for improving soil, distilling the filtrate in vacuum to separate low carbon alcohol and rough glycerin.