In the context of global climate change, how to balance economic development and environmental protection has never been more vital. In response to this challenge, low-carbon economy development is undoubtedly endorsed worldwide. This paper proposes an improved low-carbon economy development index based on data envelopment analysis (DEA) for evaluating and analyzing the low-carbon economy development process of 56 Belt and Road Initiatives (BRI) countries in 2005–2015. The results show that: (1) The average value of green total factor productivity (GTFP) in BRI countries has increased slightly, due to the technological progress and improvement of pure technical efficiency; (2) Overall, BRI countries are still in the stage of energy-extensive economy. They have not achieved a development from “high-carbon” to “low-carbon” economy development. Although the low-carbon economy development index of BRI countries shows a rising trend, there is a general decline in 2015; (3) There is no obvious correlation between the low-carbon economy development and income levels; (4) The development processes of BRI countries are different, and most of them are in an unstable initial stage of development process. In view of this, this paper proposes that (1) improve technology to increase the growth rate of GTFP; (2) change development mode due to national policies; and (3) actively promote green BRI is of vital importance to BRI countries.