The invention relates to a gas pulse stirring device under liquid and a method. The device is placed in liquid when in a working state and can comprise a box body (1), a vertical cylinder (2) and a floater (3), wherein the side wall of the box body (1) is sealed so that gas cannot escape through the side wall, the bottom of the box body (1) is a structure which the liquid can permeate through, an opening leading to the outside of the box body is formed in the top of the box body, and the box body is provided with a channel which the gas can be introduced into; the vertical cylinder (2) is located in the box body (1) and used for containing the floater, the liquid and the gas can permeate through the side wall of the vertical cylinder at least, the top of the vertical cylinder is fixed to the top of the box body, and an open mouth is formed in the top of the vertical cylinder and opposite to the opening in the top of the box body; the floater (3) is located in the vertical cylinder, the weight of the floater is smaller than buoyancy borne by the floater completely immersed in the liquid, when the box body is filled with the liquid, the upper end of the floater makes contact with the opening in the top of the box body and blocks the opening, and when the volume of the gas in the box body reaches a preset number, the floater falls from the opening. The gas pulse stirring device under liquid is simple in structure, easy to manufacture, low in manufacturing cost and easy to maintain.