BA System (Building Automation System) is an important intelligentized subsystem of Intelligent Building, also known as the groundwork of intelligent integration. It is the integration of multi-subsystems, which aims at automatic device management, efficiently saves energy and reduces use-cost. What is outstanding out of all effort for saving energy in modere Intelligent Building are both supervision and control of air-conditioner system. Now BA System comprises the basic components, here they are: central station, sub-station, spot meters, Control Network.According to survey, there are few stable BA System for long, without regard to amount of those existence. It results directly in massive equipments and related investment suspended. The thing is gone for little. The thesis illuminates functions and components of BA System thoroughly, gives further details of key technique---Control Network and System Integration, and predicts trend of this system. Based on the analysis mentioned along whith my experence, the procedure, principles and key point of BA System design and device choice are given; further more, embodies methods through illustrating real project for better utilization of BA System. In the end a conclusion and forecast of BA System are put forward.