The invention discloses a preparation method of biodiesel. The preparation method comprises the following steps: 1) carrying out transesterification on grease and fatty alcohol under the condition of a KF/MMT mixture and Na2SiO3 solid base; 2) steaming out the fatty alcohol from the mixture after the reaction and separating the glycerinum; 3) adding one or more nonionic surfactants in the mixture obtained in the step 2) to serve as filter aids, separating the fatty acid monoglyceride through a ceramic membrane to obtain the high-purity biodiesel. According to the preparation method, the process is simple, the adaptability of raw materials is relatively high, the problem of the saponified matter accumulation is avoided, the grease utilization rate can be close to 100%, the purity of the biodiesel is high, the aftertreatment is simple, the reduced pressure distillation is avoided, the concentration of the byproduct glycerinum is high, the temperature and the pressure required by the transesterification are low, the cost is reduced, the yield of the biodiesel is high, and water-bath heating is used, so that the reaction temperature is constant.