The utility model relates to a closed fuel system. The device comprises a fuel tank, a gas storage tank and a filling pipe, a pressure liquid level sensor is arranged in the fuel tank; the fuel tank is connected with a first pipeline communicated with the gas storage tank and used for high-pressure exhaust and a second pipeline for low-pressure gas supplementation. A buoyancy switch is arranged atthe bottom of the gas storage tank and connected with a third pipeline communicated with the fuel tank, the first pipeline and the second pipeline are jointly provided with a gas pump and a reversingdevice for switching the gas pump to be communicated with different pipelines, and electromagnetic valves are arranged at the ends, close to the fuel tank, of the first pipeline and the second pipeline. According to the utility model, a reliable closed system is established, the high-pressure buffer storage tank is added, steam in a fuel system is collected in time, air is supplied to or exhausted from the fuel tank in time according to the change of the pressure in the fuel tank, and all the steam is stored in the high-pressure buffer storage tank, so that recycling is realized.