Abstract: From designing practice of reinforcement and reconstruction for the old library of Southeast University,it is analysed the research category and the important of endowing old building with new mission.It is brought forward two principles of making the old repaired as the original,and the interactive in the traditional spirit and modern ideology.The two principles are better implemented by the measures of general layout,improving the value of the features,the design of transport space,structure strengthening,adding basic equipment,replacement of matching equipment,fire-fighting design,interior decoration design,energy-saving design,etc.Thus old building was endowed with new mission which,has an important practical significance in the rapid development of modern urban development.