NOVELTY - Reducing biodiesel acid value comprises rectifying crude fatty acid methyl ester or fatty acid ethyl ester with acid value of 1-5 mg potassium hydroxide (KOH)/g, adding deacidification agent occupying 0.02-4% of total weight of the fine fatty acid methyl ester or fatty acid ethyl ester, gradually warming up at less than 60-80 degrees C in stirred autoclave, stirring and reacting for 0.2-0.5 hour, placing in sedimentation tank, settling for 2 hours and releasing deacidification agent at lower layer, measuring acid value at upper layer, and placing the qualified products in finished product tank.    USE - The process is useful for reducing biodiesel acid value (claimed).    ADVANTAGE - The process is simple, where there is no need of complex equipment, and the production cost is low. Moreover, the deacidification effect is good; the produced biodiesel meets national standard that is a chemical raw material, capable of increasing economic benefits of biodiesel industries. Additionally, after several tests, measuring biodiesel acid value before and after deacidification by standard method, it is proved that the acid value of the biodiesel deacidified by the method claimed by the invention can be reduced more than 90%.    DETAILED DESCRIPTION - Reducing biodiesel acid value comprises rectifying crude fatty acid methyl ester or fatty acid ethyl ester with acid value of 1-5 mg potassium hydroxide (KOH)/g, adding deacidification agent occupying 0.02-4% of total weight of the fine fatty acid methyl ester or fatty acid ethyl ester, gradually warming up at less than 60-80 degrees C in stirred autoclave, stirring and reacting for 0.2-0.5 hour, placing in sedimentation tank, settling for 2 hours and releasing deacidification agent at lower layer, measuring acid value at upper layer, and placing the qualified products in finished product tank; where the deacidification agent is triethanolamine; and the settling matter comprises deacidification agent that is able to be repeatedly used.