Many Belt and Road Initiative (BRI) countries, being developing nations, are likely to experience rising carbon emissions due to rapid economic and social development. Promoting a low-carbon transition is essential for meeting the Paris Agreement goals. This paper employs a two-stage decomposition model to examine carbon emission trends and drivers across different income levels within BRI countries. Emission changes are attributed to four factors: economic scale, population, energy intensity and carbon intensity of energy consumption. Additionally, carbon intensity is further decomposed into coal, oil, natural gas, and non-fossil energy to assess the contributions of each energy source to emissions. Our findings reveal that while carbon emissions in BRI countries are gradually increasing, per capita emissions remain much lower than those in developed nations. Economic scale emerged as the primary driver of emission growth from 1987 to 2022, while energy and carbon intensity contributed to emission reduction. During economic crises, most BRI countries often rely on high-carbon energy for transition and relax energy efficiency standards. In low- and lower-middle-income BRI countries, energy efficiency contributes to carbon emissions growth, underscoring the significant potential for energy efficiency improvements in these regions. Notably, many BRI countries are lagging or have not yet to initiate a low-carbon transition, particularly lower-income countries, which are increasingly reliant on fossil fuels. Higher-income BRI countries have gradually shifted the energy mix from oil and coal to natural gas. Finally, recommendations are made to facilitate carbon emission reductions in BRI countries, emphasizing the importance of international cooperation.