Complete contact is often used in assembling wheel hub and shaft which are used in the closed impeller assembly group of large turbine compressor. The interference is usually fixed. In the topic, based on the nonlinear contact theory and FEM, an impeller assembly is studied from finishing assembly to steady state, then the size and distribution of stress of the contact area between wheel hub and shaft is gotten. Then a structural optimization on the assembly is done. The stress distribution is changed reasonably when the structural redundancy is reduced. Then the incidence of shaft bending is reduced effectively and assembling performance of the impeller assembly structure is improved, the results can be used to offer reference for the design and manufacture.