As global climate change worsens, the goal of reducing carbon emissions has become a global consensus. The Chinese government has already taken a series of active policy measures to reduce carbon emissions, including compensation for new energy industries and for converting gasoline-fueled vehicles to electric vehicles. Electric vehicles are gradually becoming a viable alternative to gasoline-fueled vehicles, owing to their greater efficiency and lower pollution, including lower carbon emissions. However, empirical evidence of the environmental effects of such policy measures remains unclear. For example, the coal-based grid energy structure of China may largely offset the effectiveness of carbon emission reduction policies. Meanwhile, the construction of basic ancillary facilities (e.g., the construction of charging piles, and the scrapping or recycling of old vehicles) also introduce additional carbon emission and other environmental costs, which further reduce the effectiveness of carbon emission reduction policies. As a result, at the life-cycle scale, the effect of carbon emission reduction of electric vehicles is still controversial. We systematically assessed the carbon emissions of electric and diesel-fueled buses using the life-cycle assessment method and Hangzhou City as an example, and then simulated the carbon emissions of the buses under baseline, low-carbon, and strengthened low-carbon scenarios. Our results show that:(1) At the life-cycle scale, the carbon emissions of each electric and diesel bus were 1103.237 and 1401.319 t CO₂ eq, respectively, of which electricity production and fuel combustion accounted for 74.10% and 86.96%; (2) For the currently operating 2312 electric buses in Hangzhou, the total reduction in carbon emissions at the full life-cycle (13 years) is ~0.689×10⁶ tons, or an average of 53.013×10³ tons per year; (3) It takes ~3.5 years before the total net carbon emissions of electric buses to be less than those of diesel buses; (4) For the switch of diesel buses to electric buses program at Hangzhou City, under different development of new coal technologies and improvement of energy structure scenarios, in the years of 2020, 2035, and 2050, the reduction of carbon emission of single electric bus at Hangzhou is expected to reach 354.071-884.339 ts, with an annual reduction of 27.236 (25.27%) to 68.026 t (63.11%). Particularly, under the strengthened low-carbon scenario in 2050, the life-cycle carbon emissions of electric buses will be only 46.86% and 36.89% that of current electric and diesel buses, respectively. The results of our quantitative study suggest that, at the life-cycle scale, switching from diesel buses to electric buses can provide an appealing reduction in carbon emissions.