The invention discloses a bottomless double-wall steel cover box assembling structure. The structure comprises two first double-wall plates, two second double-wall plates and supporting rods; the first double-wall plates, the second double-wall plates and the supporting rods are fixedly connected to form a rectangular box body; the supporting rods are supported on the inner side of the rectangular box body and are in cross intersection; each of the first double-wall plates and the second double-wall plates comprises at least two vertical box beams and a double-wall plate body; the first double-wall plates and the second double-wall plates are perpendicularly and fixedly connected through a plurality of connecting plates which are in bridging joint with side plates of two adjacent vertical box beams and are perpendicularly arranged in a spaced mode; pieces of water retaining structural steel are perpendicularly fixed to the four corners of the inner side of the rectangular box body respectively; the right angles of the water retaining structural steel are aligned with vertical joint seams of the first double-wall plates and the second double-wall plates. The bottomless double-wall steel cover box assembling structure is simple in structure and can be quietly conveniently assembled to form a bottomless double-wall steel cover box. A full-weld welding joint acted by shear force in the prior art is changed into local connecting welding joints formed by pull action of the connecting plates which are perpendicularly arranged in the spaced mode, the first double-wall plates and the second double-wall plates. The stress state of the connecting welding joints of the bottomless double-wall steel cover box is greatly improved, and the use reliability of the bottomless double-wall steel cover box is improved.