This paper simply states conventional and special forecasting methods, and discusses the difference between two kinds of uncertain theories, that is , stochastic and gray system one. The uncertainty of the Building Hygrothermal system (BHS) is analyzed. BHS is regarded as a gray system. Gray prediction model, GM (1,1) , is depicted. A method, Upper-Lower Limitation Lines Method (ULLLM), based on gray predicting model and gray plane is proposed. Building loads for a certain example is predicted by using the above method. By inspecting, predicting values of a certain year just fall into predicting gray plane built by UDLLM is feasible and correct. The method is important to operation and management of air-condition equipment.