The invention discloses an anti-scouring advanced pre-reinforcing device for a closed conduit below a riverbed and a design method. The device comprises a steel-stone frame and minitype steel pipe piles. The minitype steel pipe piles cling to the outer side of a framework of the steel-stone frame, and the framework of the steel-stone frame is of a right trapezoid structure overall. The framework of the steel-stone frame comprises steel-made cross beams, steel-made vertical beams and steel-made cant beams. The steel-made cross beams are arranged on the trapezoid top face and the trapezoid slopeof the framework of the steel-stone frame, the planes where the steel-made vertical beams are located are obliquely provided with the steel-made cant beams for reinforcement, and the steel-made beamsare connected through bolts. The steel-stone frame replaces an original soil body in a foundation pit to reinforce the lower portion of the riverbed, riverwater down-cutting erosion can be effectively prevented, it is avoided that the closed conduit is exposed and damaged, normal use of the closed conduit is guaranteed, meanwhile, the steel-stone frame is easy to assemble, the construction periodcan be effectively shortened, labor is saved, and the anti-scouring advanced pre-reinforcing device for the closed conduit below the riverbed and the design method can be widely applied to the fieldof closed conduit reinforcement.