Contaminants in hydraulic fluid are the primary causes of premature oil degradation. In this study, we propose a method for removing contaminants from contaminated oil. This method combines centrifugal separation and vacuum negative pressure. Centrifugal separation is utilized to separate solid particulates and free water. Vacuum negative pressure can be used to remove dissolved air. A mathematical model is developed for predicting bubble motion behavior under the complex actions of centrifugal force and vacuum negative pressure. Three performance indices that describe the purification capacity of the separator are proposed to optimise the gas-liquid-solid separator.