Abstract: The RNG k-\varepsilon turbulence model and theVOF(volume of fluid) method are adopted to solve 3D turbulent flow field inthe toilet in Fluent. The flow pattern regulation in the toilet isinvestigated. The influences of the siphon pipeline profile on speeddistributions, press distributions and siphonage performance are alsoanalyzed. The optimum hydraulic design of siphon outlet has been realizedbased on CFD flowfield analysis. It is proved that the numerical simulatedproperties of the toilet have a good agreement with the PIV test.