The Voraxial Separator (VAS) has attracted increasing attention in recent years because it can separate large volumes of oily water efficiently within a tubular compact structure. However, there is a lack of systematic research into the working mechanism and validation of the design theory of the separation technology. In this paper, the key factors influencing the separation efficiency of the VAS were analyzed, the theoretical model of this new separation technology was established, and a pilot VAS was designed. The velocity field in the pilot VAS was also analyzed by numerical simulation, which could provide a useful reference for the theoretical research. The results show that the tangential velocity distribution at the swirler end is in accordance with the pattern of Solid Body vortex, and there exists a low pressure region along the central longitudinal flowline which can be proved by the distribution of axial velocity. The maximum separation efficiency of the pilot VAS can achieve is up to 93%, which occurs when the back-flow ratio is 2.9-11.4%.