A central position of a water storage tank is provided with a drain opening; a water return bowl is in a short box shape, and is made of silicone; the bottom end of a skirt of the water return bowl is in direct contact with a bottom plane of the water storage tank to form a closed surface; a top plane of the water return bowl can attach to a grate, and the skirt of the water return bowl can be folded inwardly to form a curved flaky structure under drainage pressure; drain holes of the grate are provided on the edge of the water storage tank. Supports for supporting the grate are provided in the water storage tank. A gas buffer device is provided on the top surface of the flat-top water return bowl.