Method for preparing biological diesel oil

The biological diesel oil preparing process includes the following steps: distilling to dewater material oil to water content lower than 0.06 %; ester exchange reaction of material oil with methanol and catalyst in ultrasonic ester exchanging reactor in the molar ratio between methanol and material oil of 3-20, catalyst amount in 0.1-1 wt%, reaction temperature of 20-64 deg.c, ultrasonic frequency of 1.8 KHz to 4 MHz and ultrasonic power of 30-1000 W; distilling the ester exchange reaction product to separate methanol for reuse and separate glycerin phase and biological diesel oil phase; neutralizing, water washing and continuously distilling biological diesel oil phase to eliminate catalyst, saponified matter, water and methanol; and further purification to obtain high purity biological diesel oil. The present invention has the beneficial effects of high purity of biological diesel oil product, simple technological process, low power consumption, high conversion rate and low production cost.