By studying the growth curve of bacillus thuringiensis, the experiment takes Bt on various growth phases as "seeds" to ferment, then the curves of absorbency in 610nm and 278nm, dissolved oxygen and pH value are obtained. By comparing the curves, the best phase of Bt "seeds" is on the log phase. In comparison with unactivated Bt, the ferment cycle decreases from 32h to 28h.