Based on twoway FluidStructure Interaction (FSI) technology, a nonsteadystate model was used to get blade structural deformation of influence on distribution of flow field, pressure and flow field under the centrifugal force of inertia blade deformation and equivalent stress. Blade distortion was mainly due to the force of the media on the blade rather than the centrifugal force of inertia. Deformation function changed the pressure gradient of blade, thereby the boundary layer flow was changed to promote boundary layer separation, and to increase the vortex viscosity in flow process. The laminar flow to turbulent boundary layer was able to be promoted by blade roughness, inhibiting boundary layer separation. The major energyconsuming process of hydraulic retarder was revealed. The theoretical support for the design and verification was provided for the problems in torque and lightweight design in improving blade strength.