To address the significant energy consumption, carbon emissions, and environmental risks associated with traditional treatment methods, a novel approach is proposed for recycling shield slurries to produce a flowable synchronous grouting material. Based on the life cycle assessment (LCA) method, a carbon footprint analysis model is constructed to encompass the entire process of slurry disposal, controlled mixing, slurry preparation, and pumping grouting. A case study is conducted on a large-diameter slurry shield tunnel project of the Baifang-Fangshi Section of the Guangzhou East Railway Station-Huadu Tiangui Intercity Railway, and the carbon emissions and carbon-reduction potential of different stages are systematically evaluated. The proposed resource-utilization process for shield waste slurry meets the engineering performance requirements of synchronous grouting material, such as fluidity, setting time, and strength, while exhibiting good construction adaptability and stability. In addition, the LCA-based carbon footprint model systematically quantifies the carbon-emission contributions of each stage, among which the slurry-preparation stage accounts for the largest share of carbon emissions. Compared with traditional disposal methods, recycling the slurry as a grouting material reduces carbon emissions by 11 369.16 kg CO_2eq per ring, resulting in a total carbon-emission reduction of about 14.04%. Furthermore, sensitivity analysis indicates that the transportation distance of raw materials has the greatest impact on carbon emissions, with a sensitivity coefficient of 0.59. In addition, the working efficiency of the pumping equipment has a positive effect on carbon reduction and is a key control parameter for reducing emissions. In summary, the insitu resource utilization of shield waste slurry for preparing synchronous grouting material exhibits significant advantages in reducing carbon emissions and minimizing the scale of waste slurry disposal.