The utility model discloses a box girder strengthening structure based on steel plate-concrete and unbonded pre-stressing tendons. The box girder strengthening structure comprises a bottom steel plate, two longitudinal steel plates and two end choke plug steel plates and is located in a steel jacket box which is arranged below a strengthened area of a girder body of a strengthened box girder, and multiple unbonded pre-stressing tendons are disposed in the steel jacket box; the bottom steel plate is fixed on a bottom plate of the strengthened box girder through a plurality of first anchor bolts, the two end choke plug steel plates are fixed on the bottom plate of the strengthened box girder through a plurality of second anchor bolts, and the two longitudinal steel plates are fixed on the bottom plate of the strengthened box girder through a plurality of expansion bolts; a spiral reinforcing steel bar sleeves a tensioning end of each unbonded pre-stressing tendon, and a concrete structure is poured in the steel jacket box. The box girder strengthening structure based on the steel plate-concrete and unbonded pre-stressing tendons has the advantages that the structure is simple, the design is reasonable, the construction is simple and convenient, using effect is good, and problems of excessive concrete local stress, inadequate shear-bearing capacity and the like of tensioning anchoring ends of existing steel plate-concrete and unbonded pre-stressing combined strengthening structures can be solved.