The invention relates to a mounting method of a seabed anchor. A floating crane is provided with two sets of wire ropes, a first set of wire rope on the floating crane hoists an anchor from a square barge and moves the anchor to an area nearby a cast-anchor point, and the anchor is put into the seabed; and one end of the second set of wire rope is connected with the first set of wire rope, and the other end is connected to a floating crane, and thus a purpose of increasing the length of a hoisting rope is realized so that the anchor is thrown to a designated position; and the anchor is re-hoisted from the seabed and is suspended in seawater, and the hoisting crane is moved to the throwing point, so that the anchor can accurately reach the position of the throwing point, and the positioning of the anchor is facilitated.