The invention discloses an underwater fence board preparation and positioning installation method and an underwater fence board positioning installation system. The underwater fence board preparationand positioning installation method comprises the following steps that (1), fence board preparation is performed, specifically, a formwork is prepared, a bottom mold and side molds are installed, steel bar binding is performed, concrete is poured, mold removal is formed, fence board transportation is performed, a fence board is transported to a dock by a heavy carrying car, and the fence board islifted and placed on a transport ship by a gantry crane; and (2) the fence board is positioned and installed, specifically, the transport ship is berthed to an installation ship; a shipborne crane uses a hanging bracket to lift the fence board, and the fence board is lifted to the installation side; the lifting machine starts to lower the fence board; a scanner is turned on to display the actual real-time position of the fence board on a display screen of a main controller; the main controller controls a lifting hook to perform automatic and hydraulic unhooking, and a next fence board is installed; and an installation system includes the lifting machine, four GPS and two scanners, wherein the lifting machine, the four GPS and the two scanners are installed on a hull. The underwater fence board preparation and positioning installation method has a simple preparation process, convenient installation and positioning and high precision.