The extension-story design of a framed office building is introduced.First the dynamic characteristics of the structure with directive adding stores are analyzed,and what structural members should be reinforced are determined.Weak stiffness,low bearing capacity and detail design problems of the old frame can be found.Second,the design of anti-seismic strengthening is carried out and attention should be paid to coordination with the other departments.Finally,the dynamic characteristics of the designed structure are tested,and that satisfied the various requirements of current code.The design is one of the reinforcement programs of the project,which can be reference for other projects.