The mechanism and synthesizing method for biodiesel are introduced in detail. The different catalysts are used for the synthesizing biodiesel, such as acid, base, iodine, and oxide. The synthesizing method includes the amount of acid, the amount of load, the different soaking solution (H2SO4), the calcination's time, the amount of base, the ratio of NaOH and CaO/SiO2, the time of load, the soaking solution (KAc), the amount of iodine, the reaction temperature, and the mass ratio of KF/CaO-MgO.