The invention discloses a slipping slot type cross transfer device for assisting hoisting of a large-size box girder, which comprises two parallel cross transfer tracks, two supporting devices, two slipping slot type cross transfer girders, two pushing type lifting jacks and a lifting jack pump station, wherein the two slipping slot type cross transfer girders are respectively assembled on the two cross transfer tracks, the slipping slot type cross transfer girders and the cross transfer tracks are matched by slide slots/guide rails, and the matched positions are provided with pin holes at equal intervals and fixed by pin shafts; the two supporting devices are spanned on the two cross transfer girders and positioned on both sides of the two slipping slot type cross transfer girders; the two lifting jacks are connected with the other side of one of the supporting devices; each supporting device comprises a slipping base spanned on the two cross transfer tracks, longitudinal supporting girders are fixedly arranged on the slipping bases and arranged in parallel; and the top of the slipping slot type cross transfer device is provided with a fixed structure for stopping the movement of a transportation truck. The slipping slot type cross transfer device solves the cross transfer problem of large-size prefabricated box girders, and reduces the construction cost of girder installation engineering by about 5%-10%.