In this article, Spiral belt static mixer with changing diameter was taken as the object. The numerical simulation method was used to investigate the mixing process of two-phase liquid–liquid flow in water treatment by a commercial CFD code,namely Fluent.The k-ε model and species transport model were established to research this project. Then the mixing effect was compared with that of HEV static mixer. The result showed that spiral belt static mixer with changing diameter promote the mixing effect greatly. The mixing effect relative to that of HEV static mixer increased 10 times and the the pressure loss only increased 3 times.