Abstract: Based on self-made ionic liquid as catalyst and self-designed biodiesel preparation reaction device,biodiesel was prepared from high acid value sewage oil by pre-esterification and then esterification. The results showed that the optimum conditions were as follows:reaction temperature 105 ℃,methanol inlet rate 0.825 mL/min,catalyst dosage 5.0%,esterification rate＞95% in 2 h. Under this condition esterification rate was 77.8% after 9 times of catalyst reuse;By changing the operation method,the amount of methanol was 1.32 mL/min under pressure for 30 min,and then the amount of methanol was 0.825 mL/min under normal pressure for 30 min.Under the above reaction conditions,the acid value of trench could be reduced from 120 mg KOH/g to 0.89 mg KOH/g,the esterification rate could reach 99.26%,and the esterification time could be shortened by 1 h.