The invention relates to a buoyancy device for a blowout preventer in the deep-water well drilling water, which is characterized in that, the device comprises a blowout preventer in the well drilling water that is 80-100 m below the water level, and a buoyancy cabin connected with the blowout preventer. The buoyancy cabin is separated into a plurality of mutually enclosed chambers; an ordinary water-insolating pipe connected with the well-drilling platform passes through the center of the buoyancy cabin, and is connected with the upper end of the buoyancy cabin through an ordinary disengagingmechanism; below the blowout preventer, a ballast tank is arranged, the center of which is connected with an upper wellhead through the upper end of a stress joint, the lower end of which is connected with a hi-pressure water-isolating pipe, the perimeter of which is looped with a buoyancy block; the inside of the ballast tank is separated into a plurality of mutually enclosed chambers; in the ballast tank, a set of ordinary ballast system is arranged, which is connected with the control system of the well-drilling platform through an umbitical cable. The invention can reduce the center of gravity of the buoyancy device for the blowout preventer in the whole well-drilling system, has the advantages of the high adaptability, the good performance and the low cost, etc., and can be widely used in deep-water well drilling.