Microwave and ultrasonic are used as pretreatment technologies of sludge, because of the excellent performance on improving sludge dewaterability. This study focuses on viscosity and particle size of sludge to study by numerical simulation. The results show that the Settle Volume (SV), viscosity, and the moisture content of the sludge reduced. The increase in the Chemical Oxygen Demand (COD) concentration also indicates a significant disruption of complex floc structures. The particle size of sludge ranged from 33.55μm to 28.98μm, 9.44μm of microwave and ultrasonic pretreatment. Three-dimensional flow field in the centrifuge on the dewatering efficiency of structural parameters and sludge pretreatment are investigated numerically by using Computation Fluid Dynamics (CFD) software FLUENT. The results show that improvement of separation efficiency can be achieved by not only increasing the length-diameter ratio, but also decreasing the sludge viscosity.