NOVELTY - A solid base catalyst comprises potassium carbonate as the active component, and carrier, where it is prepared by taking potassium carbonate to be loaded to the carrier and calcinating with high temperature comprising preparing potassium carbonate to aqueous solution of 10-50%, adding carrier, making mass ratio of the carrier and the aqueous solution be 1-12:10, heating at 70 degrees C for 4 hours, distilling at 80 degrees C, baking in muffle furnace, and obtaining catalyst. The carrier is alumina, active carbon, calcium oxide, magnesium oxide, zinc oxide, zirconium oxide, kieselguhr, or pseudo-boehmite.    USE - The solid base catalyst is useful for biodiesel synthesis (claimed).    ADVANTAGE - The invention has high yield, where the catalyst is cheap and small of dosage. The reaction condition is gentle and the reaction time is short. Additionally, the catalyst can be used repeatedly, does not pollute the environment, and the demand of the material is low.    DETAILED DESCRIPTION - A solid base catalyst comprises potassium carbonate as the active component, and carrier, where it is prepared by taking potassium carbonate to be loaded to the carrier and calcinating with high temperature comprising preparing potassium carbonate to the aqueous solution of 10-50%, adding the carrier, making the mass ratio of the carrier and the aqueous solution be 1-12:10, heating up and stirring at 70 degrees C for 4 hours, distilling in rotation at 80 degrees C to remove the water content, baking for 5-10 hours at 300-1000 degrees C in muffle furnace, and obtaining the catalyst. The carrier is alumina, calcium oxide, magnesium oxide, zinc oxide, zirconium oxide, active carbon, molecular sieve, kieselguhr, argil hargil, silica gel, or pseudo-boehmite. An INDEPENDENT CLAIM is a method of synthesizing biodiesel by ester switching comprising adding oil, low-carbon alcohol and solid base catalyst to three-necked container with electromagnetic stirrer, thermometer and reflux cooling pipe to heat up, reflux and stir for 1-3 hours, controlling temperature at 50-110 degrees C, filtering the catalyst and filtrate out, separating the filtrate to separate glycerol at the lower layer, adding sodium bicarbonate (NaHCO3) solution to the mixture at the upper layer, making the adding quantity be 2-6 times of the mass of the mixture at the upper layer, washing until it is neutral, separating out organic phase at the upper layer by separating funnel, drying the organic phase with anhydrous magnesium sulfate (MgSO4), filtering to remove the drying agent, and decompressing and distilling, to obtain the transparent light yellow-like biodiesel, where the mass ratio of the oil, low-carbon alcohol and solid base catalyst is 1:0.1-0.5:0.04-0.1, the oil is vegetable oil, animal oil or the waste oil in production, or sewer tunnel oil, and the low-carbon alcohol is methanol, ethanol, propanol or butanol.