The invention discloses a rotating hanging arm device for a steel hanging box of an underwater bearing platform. The device comprises a plurality of movable hanging frames which are uniformly arranged on the top of each steel casing, and a plurality of inclined pull rods, wherein each movable hanging frame comprises a hanging frame seat and a hanging frame box; the hanging frame seat is fixed at the end of a supporting rib in each steel casing; the hanging frame box is arranged in the hanging frame seat; one end of the hanging frame box is hinged with one end of the hanging frame seat through a pin shaft; and the upper side of the other end of the hanging frame box is provided with a transverse plate fixedly connected with the two side plates of the hanging frame box. When the steel hanging box is sleeved, each hanging frame box is turned to be in a vertically upward state; when each inclined pull rod is arranged, each hanging frame box is turned to be in a horizontal state, the upper end of each inclined pull rod passes through the transverse plate and an inclined pull rod upper base plate in turn, the lower end of each inclined pull rod passes through a supporting sheath, a flat base plate and an inclined pull rod lower base plate which are arranged on a reinforcing rib at the bottom of the steel hanging box in turn, and each inclined pull rod is fixedly connected with each hanging frame box and the bottom of the steel hanging box through fastening nuts respectively. The movable hanging frames and the steel hanging box can be constructed synchronously, so the device is convenient to install, the service time of a large-scale marine crane is greatly shortened, and the construction period can be shortened by over 20 percent.