Abstract: In a constant temperature and humidity system based on excessive balance principle, air dehumidification is carried out by a refrigerator, the refrigerator and heater work simultaneously which result in low control precision, high energy consumption, and deficient fresh air supplement in laboratory.While, in a high precision and energy saving constant temperature and humidity system based on supply-demand balance principle, cooling, heating, dehumidifying and humidifying are controlled by sensors respectively.The system could run automatically, independently and intermittantly, offer higher control precision and 50 percent of energy saving comparing with the former system, and ensure fresh air in laboratory.