The invention relates to a fresh air unit and a temperature and humidity adjusting method. The fresh air unit comprises a compressor, a heat exchanger and at least two cavities. The heat exchanger is arranged in the cavities, and the cavities are provided with air inlets and air outlets. The fresh air unit further comprises at least one reversing assembly used for controlling air in the cavities to be blown into a room or out of the room. According to the fresh air unit, the reversing assembly is used for controlling cold air and/or hot air entering the room, and when cold air enters the room, the room is cooled and dehumidified; when hot air enters the room, the temperature in the room is increased; when cold air and hot air enter the room, the temperature in the room is kept unchangeable, and the dehumidifying effect is achieved. Thus, the fresh air unit can adjust the temperature and humidity in the room at the same time to meet the requirement for adjusting the indoor air temperature and humidity at different situations.