In order to research the failure of reducer, we focusedon the protective corrosion product film formed on the wall of reducer andinvestigated the mechanism of erosion-corrosion damage caused by interactionof corrosion and fluid. The mathematical model of fluid-structureinteraction was established. N-S equation of viscosity liquid and controlequation of solid area of corrosion products are postulated with ArbitraryLagrange-Euler (ALE). The interaction between multiphase liquid in boundarylayer within pipe wall and the damage of protective erodent products film isanalyzed. Applying with the Femlab software, the deformation of protectiveerodent products film in three different ways were analyzed, includingdifferent flows in reducer, different structures of reducer and differentspecifications of reducer. The numerical simulation result indicatedthat the deformation of protective erodent products film is relativelysmall, when the following conditions are satisfied, such as the fluid flowsfrom the small inlet to large outlet, the concentric reducer and therelatively small difference of two end calibers of reducer. The obtainedresults can be used in erosion protecting and technical modification for thereducer.