It provides abundant oxygen inside the room, keeps the bacteria and dust with a diameter larger than 0.3 μm outdoors, also discharges the excess carbon dioxide, organic volatiles, smoke and odors out of the room, and keeps the indoor environment as clean as the natural environment. Meanwhile, the energy can be recovered by the heat recovery system built inside the host, thereby greatly saving indoor energy. The fresh air system can regulate indoor temperature, humidity and air quality, providing a comfortable and healthy indoor environment.