Abstract: The existing steel beam described in this paper has a span of 30 m. Due to the industrial adjustment, the external load of the new curtain wall is increased, and the deflection of the steel beam is too large to meet the actual use requirements. There are two traditional reinforcement methods:one is to add steel columns under the steel beams to reduce the span of the existing steel beams, thereby reducing the deflection, but the support points under the new steel columns expand the scope of the original building reinforcement; Another is to add section steel under the existing steel beam, so as to increase the rigidity of the steel beam, but with the increase of section steel, the dead weight of the whole steel beam also increases, which is obviously not economical. At the same time, due to the tight construction period and limited construction conditions of this project, in order to solve the deflection problem of steel beams more effectively under the existing conditions, in view of the particularity of this project, it was proposed to adopt the external prestressing method to strengthen the existing steel beams. Based on the span of the steel beam and the support at both ends, the double folded line type of the prestressed steel strand was adopted. By analyzing the prestress loss of external prestressed steel strand in the process of tension, the effective tensioning prestress was obtained. Then the effective area of prestressed steel strand was obtained. By applying tension at both ends of the original steel beam and setting a steering block in the middle, the two ends of the steel beam were tensioned in batches during the installation of the curtain wall. Thus, the deflection increased by the external load was controlled by the tensioning of the prestressed steel strand. Under the combined working condition of external load and prestress, the deflection deformation was zero, which avoided the increase of deflection of existing steel beams due to the increase of new loads. The two ends of the existing steel beam were tensioned by external prestress, and the reinforcement task of the steel beam was completed in a short time. The problem of excessive deflection of the existing steel beam was solved in the limited time and on the construction operation surface, and the application requirements of the project were met. Thus, it had obtained greater economic value in a relatively short period of time. It is hoped that this paper could provide reference for peers with similar reinforcement conditions and methods, so as to achieve low-carbon, environmental protection, sustainable development of the reinforcement model.