Method for extracting residual avermectin B1a from primary crystallization mother liquor of avermectin B1a

The invention discloses a method for extracting residual avermectin B1a from primary crystallization mother liquor of avermectin B1a. The method comprises the steps that low-grade ketone is added to avermectin factice, heating and stirring are conducted to make the low-grade ketone dissolved, cooling and centrifugation are conducted, obtained supernatant is recycled and concentrated, and second factice is obtained; crystallization solvent is added to the second factice which is dissolved through heating, cooling is conducted to enable crystals to grow, the crystals are separated, and coarse powder is obtained; methanol is added to the coarse powder, then activated carbon is added for decoloring treatment, filtrate is recrystallized twice, and avermectin B1a fine powder with the content of the avermectin B1a larger than or equal to 95% is obtained. By means of the method for extracting the residual avermectin B1a from the primary crystallization mother liquor of the avermectin B1a, the B1a component in the primary crystallization mother liquor can be effectively recycled, 86% of the B1a component in the mother liquor can be recycled through the primary crystallization, and the content of the B1a component in the obtained coarse powder reaches up to 65%. In addition, the method has the advantages of being simple in process, low in cost and suitable for the needs of industrial production and meets the industrial requirements, and meanwhile the realistic problem that the factice is banned from using nationally is solved.