NOVELTY - Biodiesel prepared by Idesia polycarpa seed oil using an acid as a catalyst, is claimed. The acid is a solid acid e.g. zirconium sulfate tetrahydrate.    USE - Used as a biodiesel.    ADVANTAGE - The method: utilizes high yield and good quality of Idesia polycarpa seed oil and hence effectively reduces the raw material cost; utilizes solid acid catalyst zirconium sulfate tetrahydrate to prepare biodiesel and hence solves the problem of equipment corrosion, safety risks and environment pollution caused by the traditional strong acid; improves the recycling rate; has a short reaction time; and is economical.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for preparing the biodiesel using Idesia polycarpa seed oil, comprising: (i) hydrating and degumming by heating the Idesia polycarpa seed oil at 60-69.5 degrees C, adding heavy oil (6.1-8%) and water to the oil at the same temperature, and performing hydration degumming for 35-45 minutes, (ii) alkali refining and deacidifying by drying the degummed oil and removing water, heating and regulating the oil temperature at 65-80 degrees C, adding heavy oil (5-6%) to sodium hydroxide, sodium carbonate or potassium hydroxide with a concentration of 18-20%, stirring simultaneously, alkali refining and deacidifying for 25-39 minutes, (iii) decolorizing by refining the alkali oil at 60-80 degrees C by adding heavy oil (6-7%) and decolorizing for 30-40 minutes using activated carbon pre-activated at 160 degrees C for 1.2-2 hours, (iv) performing solid acid catalyzed reaction by adding low carbon alcohol, heavy oil (1.5-3%) to the decolorized oil in a molar ratio of 5.5:(1-10):1, adding solid acid catalyst zirconium sulfate tetrahydrate to the oil-alcohol mixture, stirring for 3-5 hours at 70-85 degrees C, where the low carbon alcohol is either methanol, ethanol or isopropanol, and (v) separating by recovering the low carbon alcohol, the solid acid catalyst zirconium sulfate tetrahydrate, glycerol and biodiesel.