In order to improve the air quality of the greenhouse and increase yield and improve quality of the products, a new system of controlling CO₂ concentration in modernized greenhouses was devoloped. In accordance with the principle of hydrokinetics, the system design fundamentals and method in detail were presented, and the system characteristics analysed. It was showed that the system was able to make up the CO₂ concentration quickly, to get well-distribution, to improve the production and economic benefits obviously, with the advantages of simple structure, low cost and well auto controlling property.