The stability of microbial flocculant (MBF) produced by Aspergillus sojae and its application to municipal thickened sludge dewatering were studied. The results showed that the MBF had high heat and acid-base endurance with high flocculating activity in a wide range of pH from 1.5 to 12. The MBF retained 96% of flocculating activity after 35 days preservation at 4℃, but in different pH the flocculating activity difference was very apparent after 35 days store at room temperature. The experimental results also demonstrated that the MBF was better than PAM and PAC in reducing specific resistance filtration. The optimal dose of MBF used for intensifying thickened sludge dewatering is 7%(volume fraction). And the more the volume of sludge is treated, the less the cost of MBF for unit volume sludge treatment.