With the continuous development of urban underground transportation systems, metro station infrastructure construction projects have increased significantly, along with their environmental costs. This paper addresses the carbon emission issues in China's metro station foundation pit infrastructure engineering by proposing an online efficient carbon emission quantification approach based on the element method. This method efficiently assesses environmental impacts and develops effective carbon reduction strategies by leveraging sub-project characteristics. Additionally, the paper introduces a lightweight, metro foundation pit element-oriented web tool, the Metro Foundation Pit Carbon Emission Calculation System (pit-MCCS), which enhances carbon emission estimation efficiency and visualization, and supports dynamic updates and multi-party collaboration. The pit-MCCS system was successfully applied to a metro station foundation pit project in Shanghai, accurately calculating total carbon emissions at 102 218.08 t. Analysis reveals that the main sources of emissions are concrete pouring of diaphragm walls, particularly the use of steel and concrete, with significant contributions from transportation and construction phases. To address high emissions, the study proposes feasible strategies, including optimizing material selection, updating machinery, and refining construction plans.