An emergy-based index system was proposed to handle the incomparability problem that occurs when quantities of different kinds must be compared in decision-making. Our results indicate that China's energy security improved over the last five decades from resource availability, resource affordability, economic efficiency, and environmental impacts perspectives. After quality adjusting of the energy it consumes, China was not yet the largest energy consumer in the world before 2016, with the emergy of its primary energy consumption increased to 99.68% that of the U.S. in 2015. The emissions of CH4 had the largest environmental impact, followed by SO2 and N2O, while the CO2 emissions is one order of magnitude lower. Speeding up the development and utilization of domestic coal bed methane (CBM) as an energy source will improve the quality and diversity of China's primary energy mix, as well as reduce CH4 emissions. China benefited from its energy imports, which accounted for about 5% of GDP over the past two decades. Increasing the fraction and diversity of coal imports can further improve the benefit ratio of energy imports to China. The emergy indices proposed here are general and can be applied to evaluate the energy security of other national systems. (C) 2019 Elsevier Ltd. All rights reserved.