The last few decades have witnessed a rapid development of green buildings in China especially the office sector. The life cycle assessment (LCA) approach has potential to weigh the benefits and costs associated with green building developments. Essentially, the LCA method evaluates the costs and benefits across a building's life cycle with a system approach. In this study, a green office building in Beijing, China, was analyzed by life cycle assessment to quantify its energy use and evaluate the environmental impacts in each life cycle stage. The environmental impacts can be reduced by 7.3%, 1.6% and 0.8% by using 30% gas-fired electricity generation, increasing the summer indoor temperature by 1 degrees C, and switching off office equipment and lighting during lunchtime, respectively. Similarly, by reusing 80% of the selected materials when the building is finally demolished, the three major adverse environmental impacts on human health, ecosystem quality, and resource depletion can be reduced by 11.3% 12.7%, and 7.1% respectively. Sensitivity analysis shows that electricity conservation is more effective than materials efficiency in terms of a reduction in environmental impacts. These findings are useful to inform decision makers in different stages of the green building life cycle.