The invention discloses a light aero-engine tail gas exhaust device and a working mode thereof. The device comprises a rotary shaft, a gas inlet pipeline and an impeller pipeline. The rotary shaft ismounted in the center of the impeller pipeline, the gas inlet pipeline is mounted on one side of the impeller pipeline in a welding mode, an exhaust pipeline used for exhausting tail gas is mounted onthe other side of the impeller pipeline in a welding mode, the rotary shaft is connected to a crankshaft used for power output through a transmission belt, and an impeller used for exhausting combustion tail gas of an engine combustion chamber is mounted in the impeller pipeline and mounted on the rotary shaft through keys. According to the light aero-engine tail gas exhaust device and the working mode thereof, by additionally arranging the exhaust device in a combustion chamber exhaust outlet, waste gas generated during explosive combustion of the combustion chamber of a light aero-engine isexhausted outwards to form a low pressure, the waste gas-air mixture exhausting speed is increased, more fresh air is retained in the combustion chamber, and it is guaranteed that secondary combustion of the light aero-engine is high in efficiency.