The utility model discloses an automatic cleaning device of natural gas flowmeters. The automatic cleaning device comprises a compressor, a gas storage tank and a cleaning agent container, wherein the gas storage tank is connected with an inlet of the cleaning agent container, an outlet of the cleaning agent container is connected with one flowmeter through a pipeline, and the cleaning agent container is used for being connected with the various different flowmeters through a variable-diameter connector. A valve is additionally arranged between the compressor and the gas storage tank, a valve is further arranged between the gas storage tank and the cleaning agent container, and a valve is also arranged between the cleaning agent container and the variable-diameter connector. The valves are all connected with an automatic control device which automatically adjusts the opening degrees of the valves. The cleaning agent container is provided with a liquid level meter which is also connected with the automatic control device, and the liquid level of the liquid level meter is reflected to the automatic control device in real time. The automatic cleaning device is combined with the automatic control technology for use, thereby achieving automatic cleaning of the gas flowmeters which are different in type and diameter.