Abstract: Because of the designing deficiencies or function change, some buildings need to be rehabilitated and strengthened. It is introduced the application of steel_concrete composite beam in rehabilitation and strengthening of a building in Beij ing. The practice has showed that using steel_concrete composite beam can achieve flexible joint treatment and larger ratio of span to height than the other beams, thus reducing the height of a floor and self_weight, as well as increasing the clear height of a flour. In addition, it can simplify the construction procedure and shorten the construction time.