Phase change material (PCM) is a thermal energy storage material with high energy storage density, which can be very useful in the applications of solar thermal energy; such as green buildings, solar thermal power generation and industrial process heat. In this paper, phase change composite was made based on nano-porous graphite and organic PCM, including fatty acids and their derivatives and molecular alloys. Firstly, by means of electrochemical methods, organic functional groups were imported onto the inner surface of nano-porous graphite, which are helpful for the nano-porous graphite to absorb large quantities of organic PCM and keep them safely in its porous space. Then, the thermal properties of the PCM/nano-graphite composite, including the phase change temperature, latent heat and thermal conductivity were studied by means of DSC tests. The nano-porous graphite enhanced the thermal conductivity remarkably, which to date had been a weak point in the applications of PCM. Additionally, the thermal cycling stability of the PCM/nano-graphite composite was observed by means of an accelerating cycling facility and DSC tests. After the processing and characterization, the PCM/nano-graphite composite was encapsulated in aluminum containers and used in the air conditioning system of a pilot green building in Shanghai China, as a tool of thermal storage for the purpose of shifting electrical load of the air conditioning system from peak periods to off-peak periods. Building materials based on the PCM/nano-graphite composite were also developed for the active and/or passive utilization of solar thermal energy in green buildings.