A development procedure of an oilfield typically undergoes three stages, including a stage at which oil production increases, a stage at which oil production stays stable, and a stage at which oil production decreases with water breakthrough. In particular, with respect to some wells after several decades of oil extraction, the approach of water injection production is commonly used, in order to improve or maintain oil production thereof. This inevitably causes abundant of water contained in produced fluids of these oil wells. For example, the moisture content in the produced fluid of some oil wells reaches 90%. Therefore, there is an urgent need of a separation device that enables efficient separation of oil and water from the oil well produced fluid.