Abstract: To rationally utilize energy, the laboratory and site experiments on the working performance of hydro-cyclones operated under the conditions of series/parallel connection are conducted in Dongsheng Company of Shengli Oilfield. The analysis. comparison and reduction on a great deal of data obtained from the experiments are carried out with mathematical statistics method, and the characteristic equations to reflect the working performance of hydrocyclones operated under the conditions of series/parallel connection are derived. The experiment steps and the characteristic equations of hydrocyclones operated under the conditions of series/parallel connection are introduced. The influence of parameters to the separation effect of hydrocyclones is analyzed.