The invention provides a method for strengthening a reinforced concrete beam through distributed external prestressing cables. Loose portions of the surface are removed, a left anchoring bolt and a right anchoring bolt are respectively and correspondingly arranged on two sides of a strengthened reinforced concrete beam, the middle bottom portion of the strengthened reinforced concrete beam is high, a steering rod notch can be installed at the middle bottom portion of the strengthened reinforced concrete beam, the cables pass through the left anchoring bolt in winding mode by adopting a continuous winding method, a cementing material is coated at the anchoring position to install an anchoring plate, the cables are connected and tightened on the right anchoring bolt through a clamping buckle, a cementing material is filled at the anchoring position to install an anchoring plate, and the anchoring plate is tightened. After the cementing material is hardened, a tension device is used for performing synchronous tension, and then the steering rod with glue is installed in the notch to support the cables. After all the cables are installed, adhesion agent is painted, polymer mortar is smeared to cover the cables and the anchoring plates, and screeding maintenance is performed. The high-strength cables are arranged in distributed mode, a vertical tension method is used, tensile force is reduced greatly, and construction is convenient. The cables are arranged in distributed mode so as to enable stress of construction members to be more reasonable.