The invention discloses a special synergist for beauveria bassiana biopesticide and a preparation method thereof, the special synergist is prepared from the following components in percentage by mass:0.05-0.5% parts of strigolactones, 1-5% parts of methanol, 5-15% parts of surfactant, 1-5% parts of filmogen, 0.5-5% parts of thickener, 3% parts of glucose, 0.5% parts of monopotassium phosphate, 0.2% parts of magnesium sulfate, 0.2% parts of calcium nitrate, and the balance of water. dissolving the filmogen, the thickener, glucose, monopotassium phosphate, magnesium sulfate and calcium nitratein water to prepare mixed liquor A; dissolving strigolactones in methanol and introducing the mixed liquid A at a stirring state, then adding a surfactant combination, after stirring, carrying out high-speed shearing for 30 min, then removing residue to obtain a finished product, the application method is to prepare the beauveria bassiana biopesticide into liquid pesticide to be applied accordingto the specification, then adding the product provided by the invention, applying the special synergist after mixing uniformly, the special synergist can significantly improve the prevention and control effect of the beauveria bassiana biopesticide, enhance the fast-acting property is enhanced and the period of preserving effect is extended.