The influence of outside air and return air on the IAQ in a typi cal air-conditioning room is analyzed. It is proved that the increasing indoor contamination makes the IAQ worse, the outside air quality has a direct effect on the IAQ, and the return air produces secondary pollutes. The theoretical formula about outside air, return air and the contaminant is induced. The simulation results show that controlling the outside air volume by contaminant mass fractin (CO₂) can save energy and improve the IAQ effectively.