Purpose <p>Hydrogen energy is regarded as one of the most promising clean energy source of the 21st century. Fuel cell electric vehicles (FCEVs) that utilize hydrogen as fuel are tackling current challenges, such as economic inefficiencies, through small-scale demonstrations, thereby contributing to deep decarbonization in the transportation sector. However, the economic value of carbon reduction achieved by FCEVs has been largely underestimated, primarily due to the absence of a scientific method to quantify the real-time carbon emission reductions of actual projects. China is currently implementing national demonstration projects for FCEVs and possesses a robust data foundation.</p> Methods <p>This paper provides a comprehensive examination of spatiotemporal scales and the characteristics of application scenarios, effectively integrating project carbon reduction methodologies with life cycle theory. It establishes a highly reliable and high-temporal-resolution generic evaluation method for carbon reduction. The paper subsequently validates this method through demonstration projects involving FCEVs in three representative regions: Beijing commuter passenger transport (BJTQ), Shanghai Baoshan Iron &amp; Steel Co. Ltd. (hereinafter referred to as “Baosteel”) transportation (SHBG), and Guangzhou municipal sanitation and environmental protection (GZSZ).</p> Results and discussion <p>The carbon emissions during the vehicle production phase of FCEVs were higher than those of baseline vehicles. During the vehicle use phase, which lasted for five days, the BJTQ project achieved the highest carbon reduction, totaling 1155.98 kg CO₂ eq. followed by the SHBG project, which reduced emissions by 37.39 kg CO₂ eq. The GZSZ project did not demonstrate any carbon reduction effects. Further research indicated that the average fuel consumption rate of the vehicles and the carbon intensity of the hydrogen used are the primary factors influencing the carbon reduction of FCVEs. In terms of carbon reduction trends, the BJTQ project began to show a carbon reduction effect on the 57·51st day of operation, while the SHBG project started to exhibit a carbon reduction effect on the 2734·15th day of operation.</p> Conclusions <p>This study successfully developed a carbon emission reduction evaluation method that integrates temporal and spatial scales with vehicle operational characteristics. The method offers a framework for harmonizing the assessment of carbon emissions from FCEVs and serves as a reference for policy evaluation and the development of carbon markets.</p>