In this paper, the straw fermentation broth was used as the medium for flocculating bacteria, inoculated with a compound microbial flocculant（Bacillus sp. CX15） and optimized the culture conditions for flocculating bacteria. The reducing sugar content of the supernatant of the bacteria was higher than that of the suspension and fermentation broth, and the reducing sugar yield of the fermentation broth was 35%, the appropriate inoculum amount of straw degrading bacteria was 1%, the optimal time for straw degradation was 6 d, and the inoculum amount of the appropriate substrate straw was 3 g. The straw saccharification broth was used as a low-cost flocculant production medium, and after 60 h of flocculation bacteria reaction, the fermentation broth was supplemented with 0.5 g/L nitrogen source to adjust the carbon to nitrogen ratio of the straw molasses. The flocculation rate was 90.4%, which showed an obvious flocculation effect on the wastewater and has good application prospects.