An automatic cleaning device and method for a range hood. The device comprises: a control assembly (10) electrically connected to an impeller (30) in a range hood, the control assembly (10) being used for detecting oil stains in the range hood, and according to the oil stain detection result, driving the impeller (30) to rotate and controlling the range hood to enter a cleaning mode; and a cleaning mechanism (20) electrically connected to the control assembly (10), the cleaning mechanism (20) spraying a cleaner and water mist on the impeller (30) when the range hood enters the cleaning mode.