Abstract: Biodiesel was prepared from waste acidificated oil ofhigh-acid-value by fore-esterification and transesterification. Fore-esterification transforms fatty acid of oil into fatty acid methyl ester(FAME) using toluene-p-sulfonic acid as catalyst. Transesterification transforms FAME into biodisel using alkali as catalyst.In the fore-esterification stage, the effects of mole ratio of alcohol to fatty acid, catalyst dose, reaction time and reaction temperature were investigated. The optimum conditions for fore-esterification are optimized by orthogonal experiment. The result shows that the optimum conditions are as follow:n(alcohol):n(fatty acid)10:1, catalyst dose 3%, reaction time 150 min, reaction temperature 70℃. The reaction time is shortened 1h and the recovery of catalyst is over 90% by the two-stage esterification. The influence of acid value of oil on transesterification was investigated. The result shows that the acid value of oil used for transesterification could not be more than 4 mg/g.