The invention provides an energy-saving fresh air system, and relates to a building internal air conditioning system which comprises a fresh air unit, a fresh air pipeline, a bypass mechanism and a tail end start-stop mechanism. The fresh air unit is provided with an air inlet end and an air outlet end. One end of the fresh air pipeline is connected to the air outlet end, the other end of the fresh air pipeline is connected to the tail end start-stop mechanism, one end of the bypass mechanism is connected to the end, close to the fresh air unit, of the fresh air pipeline, the other end of thebypass mechanism is connected to the air inlet end, and the fresh air pipeline and the air inlet end are selectively connected according to the static pressure value of the fresh air pipeline. Compared with the prior art, the energy-saving fresh air system is simple in structure, low in manufacturing cost, easy to popularize, stable, easy to maintain, and capable of greatly reducing fresh air energy consumption and saving energy.