NOVELTY - Preparation of biodiesel involves mixing dehydrated and deslagged high-acid value oil with methanol; adding a methanol solution of potassium hydroxide; adding the liquid after removing the coarse methanol to a thermal-insulating conical precipitation tank, standing and precipitating the liquid, separating the liquid for removing the glycerol and obtaining the coarse biodiesel; and respectively performing decoloring and decompression rectification treatments to the coarse biodiesel.    USE - Method for preparing a biodiesel (claimed).    ADVANTAGE - The method is simple, and has less equipment investment, low production cost, fast reaction speed, and high product conversion rate.    DETAILED DESCRIPTION - Preparing a biodiesel comprises: (A) mixing dehydrated and deslagged high-acid value oil with methanol in a mass ratio of 4:1, heating up the mixture at 58-62 degrees C, adding a composite acid catalyst (1-3 wt.%), making the mixture to pass through a static superfine dispersion device and a multi-functional refining contact reactor, continuously feeding the mixture to an esterification device, fully reacting the raw oil and the methanol in the above devices and pipelines, stopping feeding when the materials achieve two-thirds of the volume of the esterification device, sampling the materials for detecting the esterification progress, opening a circulation pump and a circulation pipeline at the bottom of the reactor for further performing reaction in a second static superfine disperser, the multifunctional refining contact reactor and a third static superfine disperser until completing the reaction when the reaction is not completed, heating up the materials by a heat exchanger when the temperature in the circulation procedure is insufficient, keeping the reaction at 58-62 degrees C; and flashing the materials for removing the coarse methanol after finishing the esterification reaction; (B) adding a methanol solution of potassium hydroxide (KOH), which is 20-30% of the raw high-acid value oil and has a concentration of 2-3%, to the liquid after removing the methanol, opening the circulation pump and the circulation pipeline, enabling the liquid to enter into the reactor through a static superfine disperser group and the multifunctional refining contact reactor to start the ester exchange reaction, heating up the liquid by an external heat exchanger when the temperature in the circulation procedure is insufficient, keeping at 58-62 degrees C, detecting the ester exchange conversion rate in the reaction procedure, and flashing the liquid again for removing the coarse methanol after ending the reaction; (C) adding the liquid after removing the coarse methanol to a thermal-insulating conical precipitation tank, standing and precipitating the liquid for 4-5 hours, separating the liquid for removing the glycerol and obtaining the coarse biodiesel; and (D) respectively performing decoloring and decompression rectification treatments to the coarse biodiesel according to the difference of the quality indexes as follows: when the acid value of the product is not greater than 0.8 mg-KOH/g and the iodine (I2) value is not greater than 80 g-I2/100 g, performing the decoloring treatment, where the terminal product is the biodiesel for vehicle and ship diesel engines, and the technical index of the product may satisfy the China BD100 standard GB/T20828-2007; and when the acid value of the product is not greater than 1.5 mg-KOH/g and the I2 value is greater than 80 g I2/100 g, performing the decompression rectification treatment, where the terminal product is an industrial chemical fatty acid methyl ester.