In this study, a new RCSF (Reinforced Concrete Steel Frame) external connection method is proposed for seismic strengtheningof medium-and low-rise reinforced concrete buildings. The RCSF method, proposed in this study, is capable of carrying out theseismic retrofitting construction while residents can live inside structures. The method is one of the strength design approach byretrofit which can easily increase the ultimate lateral load capacity of concrete buildings controlled by shear. The pseudo-dynamictest, designed using a existing school building in Korea, was carried out in order to verify the seismic strengthening effects of theproposed method in terms of the maximum load carrying capacity and ductility. Test results revealed that the proposed RCSFstrengthening method installed in RC frame enhanced conspicuously the strength and displacement capacities, and the method canresist markedly under the large scaled earthquake intensity level.