NOVELTY - One-step preparation method of bio-diesel by catalyzing oil or fatty acid by low acid comprises (a) dissolving sulfuric acid in methanol, adding oil or fatty acid and methanol sulfate in a low pressure reaction kettle; (b) recycling methanol from reaction mixture obtained in (a), resting the remnant to delaminate; (c) washing upper liquid in (b) with warm water until the lower water after delamination is neutral, adopting upper oil phase to dry and dehydrate in vacuum to obtain coarse bio-diesel; and (d) decompression distilling the coarse bio-diesel in (c), and collecting fractions.    USE - The process is useful for a one-step preparation method of bio-diesel by catalyzing oil or fatty acid by low acid (claimed).    ADVANTAGE - The process has raw material with broad application range, low acid dosage, high product transformation ratio and simple technology, it is a pressurized reaction, it can shorten reaction time, reduce energy consumption, reduce generation of three wastes pollution, and lower production coat of bio-diesel.    DETAILED DESCRIPTION - One-step preparation method of bio-diesel by catalyzing oil or fatty acid by low acid, where taking various sources and kinds of oil or fatty acid to react with methanol in the catalysis of low acid in one step to generate fatty acid methyl ester, namely bio-diesel, dosage of catalyst sulfuric acid is 1%-2% of weight of oil or fatty acid comprises (a) dissolving sulfuric acid in methanol, adding oil or fatty acid and methanol sulfate in a low pressure reaction kettle, reaction pressure is 0.2-0.3 MPa at 80-95 degrees C, catalyst sulfuric acid accounts for 1%-2% of weight of oil or fatty acid, weight ratio of alcohol to oil is 0.8:1-1.2:1, reaction time is 16-20 hours; (b) recycling methanol from reaction mixture obtained in (a), resting the remnant to delaminate, upper layer is mainly bio-diesel, lower layer is mainly glycerin and sulfuric acid; (c) washing upper liquid in (b) with 60-70 degrees C warm water until the lower water after delamination is neutral, adopting upper oil phase to dry and dehydrate in vacuum to obtain coarse bio-diesel; and (d) decompression distilling the coarse bio-diesel in (c), collecting 170-190 degrees C fractions in vacuum higher than -0.1 MPa to obtain bio-diesel product.