In the begin of 1980’s, the research of automatic control field, coupling reduction state model, was introduced into heating, ventilation and air conditioning field again after response factor method and transfer function method. It is a useful tool to analyze dynamic thermal behavior process of building. It is use to calculate heating load, cooling load and analyze energy consumption of building. The first step of this method is that the physical process is analyzed, it is to be separated subsystems simple and connected each other, then the space is descretized by numerical method, the equation (state equations and observation equations) is found up. The state model is simplified with the theory of control field, in the end all the equations of each subsystem are coupled. Now this method is very popular to settle thermal process problem of building in France