Abstract: Oil-water separator is one of new types of hydrocyclone equipment. There is a broad foreground for its application in petrochemical industry. It is important to understand the internal flow characteristics of mixed liquids. CFD is one the methods. By using the three-dimensional numerical simulation of oil-water hydrocyclone separator, some limitations and imperfections of CFD analysis were found，such as the number of the iteration, is one the significant factors influencing the simulation result.