NOVELTY - Catalyzing waste animal-plant oil and fat to produce biodiesel by solid acid, comprises: (a) removing impurity from animal and plant oil and fat or leftover animal and plant oil and fat, adding into a reactor, heating and dewatering in vacuum till the water content is less than 0.5%; (b) adding sodium acid sulfate or zirconium sulfate catalyst and methanol or ethanol thrice by adding once in every two hours into the reactor and reacting; (c) recycling remaining methanol or ethanol, adding sodium methoxide and reacting; and (d) vacuum extracting and distilling to obtain biodiesel.    USE - The method is useful for catalyzing waste animal-plant oil and fat to produce biodiesel by solid acid.    ADVANTAGE - The method: uses solid acid instead of concentrated sulfuric acid as the catalyst, when compared with the biodiesel produced by using traditional esterification method or transesterification method; utilizes less amount of organic base; causes less environment pollution and less device corrosion; reduces cost of processing sewage; saves the resource; and avoids the environment pollution problem caused by the leftovers produced by extracting the plant oil at present, and changes waste to wealth.    DETAILED DESCRIPTION - Catalyzing waste animal-plant oil fat to produce biodiesel by solid acid, comprises: (a) using animal and plant oil fat or leftover animal and plant oil and fat as raw material, removing impurity, adding into a reactor, heating to 80-90 degrees C and dewatering in vacuum till the water content is less than 0.5%; (b) adding 2% sodium acid sulfate or zirconium sulfate catalyst and 20% methanol or ethanol for three times by adding once in every two hours into the reactor, stirring and reacting for 6 hours at 110-125 degrees C with 0.2 MPa pressure to reduce acid price of the reaction product to be less than 2 mgKOH/g; (c) recycling remaining methanol or ethanol, adding 0.2% sodium methoxide, stirring and reacting for 30 minutes at 60-65 degrees C; and (d) vacuum extracting and distilling the product obtained in the step (c) at 190-290 degrees C to obtain biodiesel.