The silica sulfonic acid(SiO2-SO3H) sold acid catalysts was prepared with chlorosulfonic acid reacting with the silica prepared in sol-gel method. The catalyst was used for the transesterification of soybean and ethanol to form biodiesel. Experiments have proved that the yield of ethyl esters reached over 95.6% under the condition: treatment of catalyst at 120 ℃, molar ratio of ethanol and oil is 6∶1, the mass fraction of catalyst in oil 5.0%, the mass fraction of heptane in oil 30.0% and refluxe time 6.0 h. The results show that the catalyst so prepared has a better adaptability than the solid bases catalysts to the acidity of raw oil.