The invention relates to a method of fermenting Bt by taking blue-green algae biogas slurry as a matrix, belonging to the technical field of pesticides. The method of fermenting the Bt by taking the fresh blue-green algae biogas slurry as the matrix comprises the steps of material selection, blue-green algae biogas slurry preparation, Bt GIM1.32 culture activation, fermenting and drying. The method has the beneficial effects that 1, compared with the conventional substrates of biogas residue, Brewer's grains, corn flour, soybean cake powder, rice bran and wheat bran, the fresh blue-green algae biogas slurry can greatly lower the Bt production cost; 2, the blue-green algae biogas slurry fermented for two months is used to prepare the Bt, the finished crystal protein content of 1.87g/L, the virulence titer of 679.89IU/mu. L and LC50 0.353mL/L are all greater than those of Bt prepared under the same conditions by using the conventional culture mediums; 3, the toxin producing performance is better than that of a conventional LB culture medium as a whole, the finished product titer is high, the dispersibility is good, the quality is good, and meanwhile the recycling rate of blue-green algae is increased; 4, mass production can be performed after amplification according to a ratio.