The invention discloses a centralizing mechanical arm device for an ocean platform gate type crane. The centralizing mechanical arm device is composed of an upper connecting frame, a lower connectingframe and stand columns. The stand columns are sleeved with a sliding assembly in a sliding manner, and the stand columns are connected with crane landing legs through connecting flanges. Each of theleft side and the right side of the sliding assembly is provided with one machine frame, and each machine frame is provided with a mechanical claw. A lug plate of the upper connecting frame is fixedlyconnected with the upper end of a rack installing frame through a first pin shaft, and the lower end of the rack installing frame is in bolt connection with the lower connecting frame through a buckling pressing flange; a rack is vertically arranged on one side face of the rack installing frame; and the sliding assembly is fixedly provided with a drive mechanism, and an output small gear of a speed reducer of the drive mechanism is engaged with the rack on the rack installing frame. According to the device, mounting and dismounting are simple, and the fixed position is flexible and changeable; and the motion freedom degree of the mechanical claws is large, stability during related equipment hoisting and moving through the crane is guaranteed, and the manpower matching degree is reduced.