[Objective] This study aimed to screen the best synergistic material for Bt wettable powder and evaluate their synergistic effect. [Method] The synergism of six different kinds of additives for Bacillus thuringiensis wettable powder (Bt WP) on the 2^nd instar larvae of Plutella xylostella was tested by method of leaf dipping in labora- tory. [Result] The mixtures of Bt with 0.1% ZnCl2, 0.5% ZnCl2, 1.0% ZnCl2, 1.0% MgCI2, 0.5% boric acid, 1.0% boric acid, 0.5% citric acid or 1.0% citric acid all ex- hibited synergistic effect, in which the synergistic effect of mixture containing 0.5% boric acid was the highest, with 17.2 synergistic ratio; followed by the mixture containing 1.0% ZnCl2, with 15.6 synergistic ratio. Moreover, addition of 0.5% boric acid could shorten the median lethal time of Bt wettable powder by about 10 h. After the mixtures of Bt with 0.5% boracic acid or 1.0% ZnCl2 was stored for 15 d at room temperature, toxicities of the two mixtures did not change significantly. [Conclusion] Boracic acid as the synergist of Bt wettable powder could not only increase insecti- cidal effect of Bt, but also accelerate its insecticidal rate. So, boracic acid could improve the disadvantages of Bt wettable powder such as poor insecticidal effect and slow insecticidal speed in a certain degree.