The invention discloses an ultrasonic drainage device for a horizontal well. The ultrasonic drainage device for the horizontal well comprises a body, the body is in a cylinder shape, a dispenser is fixedly connected to one end of the body, and an ultrasonic transducer is fixedly connected to the other end of the body; the ultrasonic transducer is sequentially connected with a signal generator, an energy saving controller, and a liquid level detection device through wires; the signal generator, the energy saving controller and the liquid level detection device are all fixedly connected with the body; and a first water inlet hole is formed in the ultrasonic transducer, a second water inlet hole is formed in the liquid level detection device, a detector is arranged in the liquid level detection device, and a air hole is formed in the lateral walls of the body. According to the ultrasonic drainage device for the horizontal well, the liquor-carting volume of gas can be adjusted according to liquor-accumulating volume of a gas well. The invention further discloses a drainage method by using the ultrasonic drainage device for the horizontal well.