NOVELTY - Preparing biodiesel, comprising (i) removing food waste oil and solid impurities in catering waste oil; (ii) adding activated clay into the catering waste oil, heating, centrifuging, and dehydrating; (iii) adding catering waste oil into a reaction container; (iv) adding ethanol as an ester exchange agent into the solution, and mixing; (v) adding catalyst, namely p-toluenesulfonic acid into mixed solution; (vi) stirring mixed liquid by carrying out transesterification reaction to obtain mixture of bio-diesel and by-products e.g. glycerol; (vii) separating bio-diesel, washing, and drying.    USE - The method is useful for preparing biodiesel (claimed).    ADVANTAGE - The method overcomes: the utilization of sulfuric acid as a catalyst to prolong the reaction time, under high temperature conditions, the production of poor quality of byproduct, corrosion of equipment, generation of large amount of acidic waste water, environmental pollution, and utilization of an alkali as a catalyst adding acid value to the waste oil; saves the saponification reaction in order to prevent the pre-processing of acid according to requirements; and produces product with high activity, and does not easily cause side effects.    DETAILED DESCRIPTION - Preparing biodiesel, comprising (i) removing food waste oil and solid impurities in catering waste oil used as raw material; (ii) adding activated clay into the catering waste oil, heating, centrifuging, and dehydrating; (iii) adding catering waste oil into a reaction container; (iv) adding ethanol as an ester exchange agent into the solution placed in the reaction container, and mixing; (v) adding catalyst, namely p-toluenesulfonic acid into the obtained mixed solution; (vi) stirring the mixed liquid by carrying out transesterification reaction at 80-90 degrees C to obtain mixture of bio-diesel and by-products e.g. glycerol; (vii) separating bio-diesel from the mixture, washing with water, and drying to obtain pale yellow, clear and transparent liquid bio-diesel product.