NOVELTY - Preparing biodiesel by catalyzing swill-cooked dirty oil with solid super strong acid, comprises: (a) filtering, degumming, dehydrating and decoloring swill-cooked dirty oil; (b) adding pre-treated swill-cooked dirty oil and methanol into a reaction kettle, and stirring while adding solid super strong acid catalyst, heating, and carrying out esterification reaction, and then cooling, filtering and removing the solid super strong acid catalyst; and (c) adding water by stirring, refining and distillation under reduced pressure, cooling, decoloring and filtering by active carbon.    USE - The method is useful for preparing biodiesel by catalyzing swill-cooked dirty oil with solid super strong acid.    ADVANTAGE - The method utilizes solid super strong acid catalyst for esterification, which does not corrode apparatuses, has high catalytic activity, can be easily separable from the reaction system and recyclable, and does not produce environmental pollution. The process of adding water in the method for refining and distillation under reduced pressure to reduce the content of secondary products (with low boiling point) and residual acid in the product, and improves integral performance of the product.    DETAILED DESCRIPTION - Preparing biodiesel by catalyzing swill-cooked dirty oil with solid super strong acid, comprises: (a) filtering, degumming, dehydrating and decoloring swill-cooked dirty oil; (b) adding pre-treated swill-cooked dirty oil and methanol into a reaction kettle with mass ratio of 1:1, and stirring while adding solid super strong acid catalyst, heating, and carrying out esterification reaction at 70-80 degrees C for 3-5 hours, meanwhile instantly eliminating secondary products of glycerine and water in the reaction, and then cooling to 60 degrees C, filtering and removing the solid super strong acid catalyst; and (c) adding water by stirring, then refining and distillation under reduced pressure, cooling to 80 degrees C, decoloring and filtering by active carbon to obtain biodiesel.