China has adhered to the concept of green development since the introduction of the Belt and Road Initiative in 2013. However,as the initiative continues to advance,speculations have been raised on the possibility of its reduction of the environmental quality of the countries along the routes. So,has the seven-year-old Belt and Road Initiative reduced the environmental quality of these countries and how does it affect the environmental quality? To address these issues,this study conducted a quasi-natural experiment,based on the WDI 2005-2017 data,the human development index database of the World Bank and the OECD (Organization for Eco-nomic Cooperation and Development) database. A country-year panel data was constructed using the difference-in-differences model to assess the countries along the\'area \' initiative for environmental effects,and to further examine the heterogeneity and the policy effect mechanism. The main conclusions were as follows:① The Belt and Road Initiative had significantly improved the environmental quality of countries along the routes. The conclusions remained robust after the introduction of the placebo test, the use of instrumental vari-ables to alleviate possible endogenous problems and a series of other robustness tests.②The results of hetero-geneity analysis showed that the Belt and Road Initiative exerted a greater effect on the countries with lower environmental quality than those with higher quality in terms of environmental improvements.③Mechanism analysis showed that the Belt and Road Initiative improved the environmental quality of countries along the routes by promoting technological progress and strengthening environmental regulation, but it failed to signifi-cantly improve the environmental quality of these countries by promoting clean energy structure and indus-trial structure and increasing the scale effect. Therefore,China should continue to vigorously promote the con-cept of developing a green Belt and Road, expand the coverage of the green Belt and Road,improve the level of clean energy structure of the Belt and Road countries through investment and cooperation,and promote the clean transformation of their industrial structure. These findings provide empirical evidence and implications for the better promotion of the green Belt and Road Initiative,and have certain reference value for the follow-up implementation of the Belt and Road Initiative.