Many building simulation programs have been developed all over the world since the computer-aided simulation technology was first applied in the 1960s. In early 1980s, Tsinghua University has started to develop a new building simulation tool DeST with the aims to benefit for practical and research use of building simulation related applications in China. DeST can be used to simulate and analyze both building energy consumption and HVAC (heating, ventilation and air-conditioning) system. It has been designed to aim improving the reliability of system design, to ensure the quality of the system performance, and to reduce energy consumption of buildings. This paper reviews the development history, state-of-the-art on the development of building simulation technology and introduces the main objective, structure, and core programs of DeST. The analytical verifications, inter-program comparisons, and empirical validations of DeST are also presented in this paper. The application of DeST will be introduced in part Ⅱ of the companion paper.