Belt and Road Initiative (BRI) countries are major energy producers and consumers in the world, and they have enormous potential for energy cooperation, energy saving, and CO2 emissions reduction due to their various resource endowments. However, little quantitative research has been conducted under the BRI in the same framework. Therefore, by developing a data envelopment analysis optimisation model combined with the window analysis method, this paper investigates the energy performance of BRI countries for the period from 1995 to 2015, and evaluate the potential of energy saving and CO2 emissions reduction for each BRI country. The results show that, first, the average energy performance of 56 BRI countries is about 0.69, with evident difference across regions and countries. Specifically, in Sub-Saharan Africa and Europe and Central Asia, energy performance is relatively lower, and their averages are 0.59 and 0.60, respectively; in particular, Ukraine has the lowest energy performance among the 56 BRI countries (0.24); while the energy performance in Middle East and North Africa and South Asia appears relatively higher (0.80 and 0.89, respectively). Second, these 56 BRI countries have great energy saving potential, about 9.95 billion metric tonnes of oil equivalent from 1995 to 2015. Among them, Europe and Central Asia, East Asia and Pacific, and Middle East and North Africa make relatively larger contribution. Finally, these 56 BRI countries may produce potential CO2 emissions reduction of 50.87 billion metric tonnes during the study period, and Europe and Central Asia and East Asia and Pacific contribute the most (45.18% and 25.53%, respectively).