To explore how renewable energy reduces carbon dioxide (CO₂) emissions, this study decomposes the impacts of renewable energy on CO₂ emissions by employing the Generalized Divisia Index Method (GDIM), based on panel data of 25 countries along the Belt and Road Initiative (BRI) for the period 2005–2019. Also, we explore their decoupling relationships. The results indicate that: (1) The numbers of countries with an ideal strong decoupling state reach the highest point during 2010 and 2015. (2) The growth in renewable energy scale is the key driving force responsible for promoting CO₂ emissions in most countries along the BRI, while the main factor inhibiting CO₂ emissions is the carbon intensity of renewable energy. (3) The carbon mitigation effect of the increase in the proportion of renewable energy is considerably higher than that of technological improvements from renewables, and it will promote the transformation of the relationship between renewable energy and carbon emissions into a more ideal strong decoupling relationship. (4) The effect of carbon efficiency on emission is not significant at this stage, but it will be much more important in the future.