The influences of fermentation synergist on the avermectin fermentation titer in the production of avermectins were studied. The test results showed that a certain amount of fermentation synergist was added in the fermentation medium, the hyphae knot ball state could be improved, the concentration of hyphae was increased, the enzyme activity related with sugar metabolism could be stimulused, thereby the fermentation titer was improved,when adding 1.6×10-4 fermentation synergist,the fermentation titer could be increased up to 6.64%.