Abstract: The seismic analysis and design of safety-related concrete structure of a nuclear power station are mainly based upon linear elastic assumption，without consideration of the non-linear features of the material and the bond-slip effect between steel reinforcement and concrete． Actually，after years of operation of a nuclear power station，its aging structure has been of the elastic-plastic feature gradually obvious． Taking a two-story experimental frame as its study object as well as adopting fibre beam element，it was calculated floor response spectra by using linear elastic model and elastic-plastic model respectively and then a comparison between the tested value was done． The results showed that elastic-plastic feature should be fully considered in the seismic response evaluation of nuclear power station in operation，so as to reasonably evaluate structure seismic response and floor response spectra，thus providing a valid basis for seismic re-evaluation of nuclear power equipment，system and component． In China，it's been 20 years since the earliest nuclear power stations have been put into operation，the analytic methods and conclusion of this paper would serve as useful reference for seismic safety evaluation of nuclear power stations in operation．