NOVELTY - Preparing biodiesel by providing waste oil of cooking and food as raw materials, comprises e.g. removing impurity from the waste oil to obtain crude oil; feeding the crude oil into a first reaction kettle for dehydrating and feeding it into a second reaction kettle, adding methanol and lipase catalyst and esterifying; feeding it into a third reaction kettle and stirring; heating to distill out the lipase; washing with water; feeding the crude biodiesel into a fourth reaction kettle, precipitating and draining out the lower water layer; and heating the upper-layer in vacuum.    USE - The method is useful for preparing biodiesel.    ADVANTAGE - The method: yields high-purified and good-quality biodiesel; utilizes cooking waste oil to obtain the biodiesel having purity of above 99.5 %; gives byproducts which can be recycled as fertilizers without secondary pollution; employs lipase as the catalyst for esterification which is eco-friendly; achieves 99 % of environmental protection rate; transforms the non-usable waste oil into eco-friendly biodiesel; and turns the waste materials into usable material without causing pollution, thus purifying the environment.    DETAILED DESCRIPTION - Preparing biodiesel by providing waste oil of cooking and food as raw materials, comprises: (i) removing impurities from the waste oil to obtain crude oil; (ii) passing the crude oil into a first reaction kettle in a vacuum state at 70-110 degrees C for dehydrating; (iii) passing the dehydrated oil into a second reaction kettle, adding methanol (15-25 wt.% of crude oil) and lipase catalyst (2-3 wt.% of crude oil) into the second reaction kettle and performing esterification to obtain a crude biodiesel; (iv) passing the esterified crude biodiesel into a third reaction kettle and stirring at 70-80 degrees C until no methanol is steamed out; (v) heating the recycled methanol crude biodiesel at 80-100 degrees C and stirring to distill out the lipase; (vi) washing the recycled lipase crude biodiesel with water (15-25 wt.% of crude oil); (vii) feeding the washed crude biodiesel into a fourth reaction kettle, precipitating for 4-5 hours for layering to obtain a upper-layer and lower layer of crude biodiesel and draining out the lower water layer; and (viii) heating the obtained upper-layer of crude biodiesel in vacuum to 180-320 degrees C to precipitate crude biodiesel and distilling to obtain a purified biodiesel product.