The invention discloses an energy-saving building HVAC system, comprising a floor detection unit, an external environment detection unit, a natural wind conveying opening and closing control unit, an outdoor air quality detection unit, an indoor air quality detection unit, an air filter unit, and a harmful gas detection unit. Unit, natural wind ventilation control host, floor main ventilation duct delivery module, floor ventilation branch duct opening and closing module, floor ventilation branch duct delivery module, temperature analysis control module, active temperature regulation unit, cold source equipment, heat source equipment, floor information Acquisition unit, human flow detection module, human flow analysis module, area division module, wind direction control host, and air conditioner outlet direction reversal module. The air temperature adjustment required by different areas is analyzed through the human flow analysis module, and then the air volume is divided into different areas by the area division module, thereby reducing unnecessary HVAC air output and achieving energy-saving effects.