In China, the reinforced concrete buildings built during the period of the Republic of China have been used for so long a time that they now have different levels of damages and need to be strengthened and repaired urgently. Because most of these buildings are landmarks or protected historic buildings that bear historical information and culture value, they should be strengthened with preservation of historical information. In order to scientifically preserve these buildings, some typical cases were tested and investigated on site. The common defects of these buildings were analyzed, and the strengthening methods for different defects of concrete columns, beams and slabs were developed based on the analysis of common defects and preservation principles.