NOVELTY - A heteropoly acid ionic liquid comprises heteropoly acid and acidic ionic liquid. The heteropoly acid is molybdovanadophosphoric acid. The acidic ionic liquid is hydrogen sulfate.    USE - Heteropoly acid ionic liquid is used as catalyst for transesterification reaction to produce biodiesel (all claimed).    ADVANTAGE - Biodiesel is prepared with high yield using the heteropoly acid ionic liquid by economical and quick process.    DETAILED DESCRIPTION - A heteropoly acid ionic liquid comprises heteropoly acid and acidic ionic liquid. The heteropoly acid is molybdovanadophosphoric acid (H4PMo11VO40, H5PMo10V2O 40 or H6PMo9V3O40). The acidic ionic liquid is hydrogen sulfate 3-methylimidazolium hydrogen sulfate. INDEPENDENT CLAIMS are included for the following:    (1) preparation of heteropoly acid ionic liquid, which involves adding 8.2 g N-methylimidazole to a three-hole bottle, cooling in ice water bath, stirring for 30 minutes, adding 10-12 g 98% concentrated sulfuric acid and 10 mL water, continuously stirring at room temperature for 2-4 hours, reacting at 75 degrees C, removing excess water, obtaining acidic ionic liquid, preparing molybdovanadophosphoric acid (H4PMo11VO40, H5PMo10V2O 40 or H6PMo9V3O40), mixing acidic ionic liquid and molybdovanadophosphoric acid, adding ethanol as solvent, stirring at 50-80 degrees C for 15-30 hours, dividing resultant product into two layers, removing lower layer, and drying the upper layer in a vacuum dryer. Molybdovanadophosphoric acid (H4PMo11VO40) is prepared by adding 3.58 g disodium phosphate decahydrate and 50 mL distilled water to a beaker, adding 26.65 g sodium molybdate and 60 mL distilled water, mixing, heating, obtaining reaction solution (a1), mixing 0.91 g vanadium pentoxide and 10 mL sodium carbonate solution having concentration of 0.5-2 mol/L, mixing obtained solution and reaction solution (a1), stirring, heating to 70-100 degrees C, adding sulfuric acid solution until pH value of resultant solution is 2, cooling to room temperature, obtaining mixed solution, adding 50 mL diethyl ether, stirring, adding sulfuric acid solution, stirring for 5-10 minutes, removing diethyl ether, adding water, placing resultant product in a drying box, precipitating, crystallizing, and drying. Molybdovanadophosphoric acid (H5PMo10V2O 40) is prepared by adding 3.58 g disodium phosphate decahydrate and 50 mL distilled water to a beaker, adding 24.2 g sodium molybdate and 60 mL distilled water, mixing, heating, obtaining reaction solution (a2), mixing 2.28 g vanadium pentoxide and 10 mL sodium carbonate solution having concentration of 0.5-2 mol/L, mixing obtained solution and reaction solution (a2), stirring, heating to 70-100 degrees C, adding sulfuric acid solution until pH value of resultant solution is 2, cooling to room temperature, obtaining mixed solution, adding 50 mL diethyl ether, stirring, adding sulfuric acid solution, stirring for 5-10 minutes, removing diethyl ether, adding water, placing resultant product in a drying box, precipitating, recrystallizing, and drying. Molybdovanadophosphoric acid (H6PMo9V3O40) is prepared by adding 3.58 g disodium phosphate decahydrate and 50 mL distilled water into a beaker, adding 21.8 g sodium molybdate and 60 mL distilled water, mixing, heating, obtaining reaction solution (a2), mixing 3.2 g vanadium pentoxide and 10 mL sodium carbonate solution having concentration of 0.5-2 mol/L, mixing obtained solution and reaction solution (a2), stirring, heating to 70-100 degrees C, adding sulfuric acid solution until pH value of resultant solution is 2, cooling to room temperature, obtaining mixed solution, adding 50 mL diethyl ether, stirring, adding sulfuric acid solution, stirring for 5-10 minutes, removing diethyl ether, adding water, placing resultant product in a drying box, precipitating, recrystallizing, and drying; and    (2) usage method of heteropoly acid ionic liquid as catalyst in transesterification reaction.