Nowadays, building energy consumption has been regarded as the biggest part of national energy consumption. The reduction of air-conditioning energy consumptions is considered to be an effective way to improve the energy efficiency in buildings. To this end, software needs to be developed to evaluate the thermal performance of buildings. In this paper, a practical tool called TongJi Curtain Wall (TJCW) is developed to compute the thermal performance of glass curtain walls based on software secondary development. The mathematical model of heat transfer in glass curtain walls is first described. Then, a detailed design procedure of this tool is presented. Finally, the developed tool is validated by an example. Results indicate that this tool could be used for the calculation of thermal performance of glass curtain walls and will promote the study of building energy efficiency.