The invention provides a device for testing air quantity of fan and a method thereof. The device for testing air quantity of fan comprises an air duct, an instructing unit and a sensing unit, and is internally provided with a movable object to be tested; the air duct is arranged at an air outlet; the sensing unit is arranged on the air duct, coupled with the instructing unit and used for sensing the movable object to be tested; when the sensing unit senses the movable object to be tested, the sensing unit controls the instructing unit to generate instruction information. The invention utilizes the sensing unit to sense the movable object to be tested and control the instructing unit to generate instruction information, therefore, a tester can know whether the fan rotates and the air quantity reaches the minimum permissible air quantity or not without observing on site at any time.