The effects of the mixing of the two kinds of sludge in the agitated tank were analyzed with the activated sludge and digested sludge as the research objects.Ansys Fluent 15.0 was used to simulate the performance of the jet mixing device under the condition of the incident pressure of 226043 Pa.The incident flow rate of the activated sludge was 314.57 m3 ? h-1, the incident velocity was 3.37 m ? s-1, the digested sludge flow rate was 312.05 m3 ? h-1, the incident velocity was 3.20 m ? s-1.The simulation results show that the mixing effect is not affected by the nature of the fluid in the liquid-liquid nozzle inside, and all of them are well stirred and mixed.But in the stirring tank activated sludge mixing and mixing effect is better than digestive sludge,the mean flow velocity of the activated sludge in the stirred tank was 144.24, the volume fraction of the dead zone was 13.33%, the average flow velocity was 0.1082m ? s-1, the average velocity of the digested sludge in the stirred tank was 0.0318 m ? s-1