Many factors can have effect on flow field and performance, which makes the hydrocyclone design a complex work. Liquid’s viscosity is one of these factors, but it is costly and almost impossible to investigate how the liquid viscosity influences the performance of the hydrocyclone by test or experiment. At present, CFD technique provides us a convenient method for the flow field research in the hydrocyclone. This paper, intended to explore the effect of liquid’s viscosity on the flow field and hydrocyclone’s efficiency, presents a simulation to model the flow field in the hydrocyclone by changing the viscosity of the liquid. By comparing the results of the simulation, it is found that liquid with higher viscosity can reduce the velocity of the flow because of the friction between flow and wall and the friction between the two phases. Furthermore, viscosity increase of the liquid enlarges the area of shot flow and circulating flow, which make separating efficiency decrease and energy consumption rise.