NOVELTY - A solid acid catalyst for producing biodiesel, where the degree of sulfonation of sulfonated phenyl cerium pamidronate is 100-200%.    USE - The solid acid catalyst is useful for producing biodiesel (claimed).    ADVANTAGE - The solid acid catalyst produces biodiesel with excellent environmental impact, low discharge of sulfide, does not contain aromatic hydrocarbon, low waste gas to human and environment; capable of biodegradation, no oil pollution; can start under minus 20 degrees C; provided with good lubricating performance, which can reduce the oil injection pump, the engine cylinder body of the abrasion and prolongs the service life; has high flash point, and the biodiesel has good safety performance.    DETAILED DESCRIPTION - A solid acid catalyst for producing biodiesel, where the degree of sulfonation of sulfonated phenyl cerium pamidronate of Formula (I) is 100-200%.    X=0 or 1.    An INDEPENDENT CLAIM is a preparation method of the solid acid catalyst for preparing biological diesel, comprising: (1) preparing sulfonated benzene phosphoric acid: adding liquid sulfur oxide (SO3) slowly into a round flask containing phenylphosphonic acid and 1,2-dichloroethane mixed solution, controlling the temperature at -4 to 0 degrees C, allowing the solution to gradually changing from colorless to yellow, then heating and refluxing for a period of time to obtain double-phase liquid, where the upper layer is colorless 1,2-dichloroethane and the lower layer is yellow and thick sulfonated phenyl phosphate crude product, removing the upper layer clear liquid, and dissolving the obtained crude product into de-ionized water, adding barium chloride (BaCl2) solution to remove the unreacted SO3 and sulfate ion in the reaction mixture, centrifuging to remove the barium sulfate (BaSO4) precipitate, adding the filtrate to the cation exchange resin for ion exchange, filtering, concentrating the obtained filtrate with a rotary evaporator and then extracting again with ether repeatedly to remove un-reacted benzene phosphonic acid till the pH of the ether solution after extracting is 7, drying the extracting residual to obtain yellow thick sulfonated phenyl phosphate; (2) preparation of sulfonated-cerium phosphate: adding solid ammonium ceric nitrate to the solution beaker holding dilute nitric acid solution, stirring till completely dissolved, adding the solution prepared in the step (1) to de-ionized water solution of sulfonated phenyl phosphate, where the solution immediately changes into yellow emulsion, then heating, stirring and repeatedly washing the centrifuged deposition, drying to obtain the light yellow powder product which is sulfonated phenyl phosphate.