Building consumes more than 30% of total energy in the city to supply air-conditioning, hot water and other service, especially in the mega cities. Building energy has become the no.1 in energy consumption growth list for most developing cities. In this paper, a real building energy system which includes heating, cooling, dehumidification, and PV and wind power generation, etc., is investigated based on the practical load of a green energy building in Shanghai Jiao Tong University. The unique feature of this energy system is that it can harvest the renewable energy and the ambient energy from solar radiation, ground, air or river water sufficiently and have the minor influence to the environment. This will also lead to a low carbon emission. The system consists of such sectors, as solar thermal energy collection, PV power production, solar adsorption cooling, desiccant dehumidification, water / ground resource heat pumps, etc. A comprehensive system model will be developed, and a case study on the balance of energy structure for this green building will be made. The work is helpful towards a sustainable energy supplying structure for a practical building.