The invention discloses a device and method for joint reinforcing through prestressing, mechanical anchoring and an external bonding carbon fiber plate. The device comprises a fixing end base and a tensioning end base which are fixed to the surface of a to-be-reinforced structure. The carbon fiber plate is fixed to the fixing end base and the tensioning end base. A tensioning system used for tensioning the carbon fiber plate is installed at the tensioning end. The contact surfaces of the to-be-reinforced structure and the carbon fiber plate are bonded through a chemical adhesive. Meanwhile, the to-be-reinforced structure is further anchored through a mechanical anchoring structure. In order to give full play to the high-strength performance of the carbon fiber plate, pre-stressing force is applied to the material, so that the material gives play to certain strength in advance; and in order to enhance the anti-stripping performance of the carbon fiber plate, the carbon fiber plate is externally bonded to the surface of the to-be-reinforced structure and mechanical anchoring measures are applied, the carbon fiber plate is made to work cooperatively with the to-be-reinforced structure, and the device and method have the optimal reinforcing effect for a reinforced concrete structure.