The invention discloses a design method for unbonded prestress and steel-concrete combined reinforcement of a concrete box girder. The design method comprises the following steps of I, determining a combined reinforcing structure: determining the unbonded prestress and steel plate-concrete combined structure of the box girder to be reinforced wherein the combined reinforcing structure comprises a steel jacket box formed by a base steel pate, two longitudinal side steel plates and two end part plugging steel plates, and a plurality of unbonded prestressed reinforcing bars are arranged in the steel jacket box; II, determining structural parameters for reinforcing the front box girder; and III, determining structural parameters of the combined reinforcing structure in the process comprising the following steps of initializing the parameters, calculating theoretical maximum thickness of the reinforcing steel plates, determining the thickness of the reinforcing steel plates and the thickness of concrete, and determining the height of the longitudinal side steel plates and the width of the base steel plate. The method disclosed by the invention is simple in steps, reasonable in design, convenient to realize, and good in using effect, the structural parameters of the combined reinforcing structure of a bridge can be simply, conveniently and quickly determined, and the designed combined reinforcing structure of the bridge is economical and practical and good in reinforcing effect.